Background RSV is recognized as the most important cause of serious lower respiratory tract illness in infants and young children worldwide leading to hospitalisation in a great number of cases, especially in certain high-risk groups. The aims of the present study were to identify risk groups, outcome and incidences of hospitalisation for RSV bronchiolitis in Norwegian children under two years of age and to compare the results with other studies.
Background
RSV is recognized as the most important cause of serious lower respiratory tract illness in infants and young children worldwide leading to hospitalisation in a great number of cases, especially in certain high-risk groups. The aims of the present study were to identify risk groups, outcome and incidences of hospitalisation for RSV bronchiolitis in Norwegian children under two years of age and to compare the results with other studies.Methods We performed a population-based retrospective survey for the period 1993–2000 in children under two years of age hospitalised for RSV bronchiolitis.
Methods
We performed a population-based retrospective survey for the period 1993–2000 in children under two years of age hospitalised for RSV bronchiolitis.Results 822 admissions from 764 patients were identified, 93% had one hospitalisation, while 7% had two or more hospitalisations. Mean annual hospitalisation incidences were 21.7 per 1.000 children under one year of age, 6.8 per 1.000 children at 1–2 years of age and 14.1 per 1.000 children under two years of age. 77 children (85 admissions) belonged to one or more high-risk groups such as preterm birth, trisomy 21 and congenital heart disease. For preterm children under one year of age, at 1–2 years of age and under two years of age hospitalisation incidences per 1.000 children were 23.5, 8.7 and 16.2 respectively. The incidence for children under two years of age with trisomy 21 was 153.8 per 1.000 children.
Results
822 admissions from 764 patients were identified, 93% had one hospitalisation, while 7% had two or more hospitalisations. Mean annual hospitalisation incidences were 21.7 per 1.000 children under one year of age, 6.8 per 1.000 children at 1–2 years of age and 14.1 per 1.000 children under two years of age. 77 children (85 admissions) belonged to one or more high-risk groups such as preterm birth, trisomy 21 and congenital heart disease. For preterm children under one year of age, at 1–2 years of age and under two years of age hospitalisation incidences per 1.000 children were 23.5, 8.7 and 16.2 respectively. The incidence for children under two years of age with trisomy 21 was 153.8 per 1.000 children.Conclusion While the overall hospitalisation incidences and outcome of RSV bronchiolitis were in agreement with other studies, hospitalisation incidences for preterm children were lower than in many other studies. Age on admission for preterm children, when corrected for prematurity, was comparable to low-risk children. Length of hospitalisation and morbidity was high in both preterm children, children with a congenital heart disease and in children with trisomy 21, the last group being at particular high risk for severe disease.
Conclusion
While the overall hospitalisation incidences and outcome of RSV bronchiolitis were in agreement with other studies, hospitalisation incidences for preterm children were lower than in many other studies. Age on admission for preterm children, when corrected for prematurity, was comparable to low-risk children. Length of hospitalisation and morbidity was high in both preterm children, children with a congenital heart disease and in children with trisomy 21, the last group being at particular high risk for severe disease.

Background Respiratory tract infections due to respiratory syncytial virus (RSV) are very common in young children worldwide. In temperate climates the infection occurs in yearly winter epidemics, and by two years of age most children have been infected [ 1 ]. Reinfections are common throughout life but the first infection is usually the most severe [ 2 ]. Symptoms vary from a mild upper respiratory tract infection to a severe bronchiolitis with hyperinflated lungs and hypoxemia [ 3 ]. Children in the first months of life, particularly those with preterm birth, underlying chronic lung disease (CLD), congenital heart disease (CHD), neuromuscular disease, airway malformations or impaired cellular immunity are at risk for severe disease [ 4 - 6 ]. Local epidemiological studies are important when considering new strategies for the prevention of RSV bronchiolitis. An example of such a strategy is the monthly injection of the humanised murine monoclonal antibody palivizumab to certain high-risk groups, which in many studies has shown a significant reduction in the incidence of hospitalisation for RSV bronchiolitis [ 7 ]. The aims of the present study are to identify risk groups, outcome and incidences of hospitalisation for RSV bronchiolitis in Norwegian children under two years of age, and to compare these results with other studies.
Background
Respiratory tract infections due to respiratory syncytial virus (RSV) are very common in young children worldwide. In temperate climates the infection occurs in yearly winter epidemics, and by two years of age most children have been infected [ 1 ]. Reinfections are common throughout life but the first infection is usually the most severe [ 2 ]. Symptoms vary from a mild upper respiratory tract infection to a severe bronchiolitis with hyperinflated lungs and hypoxemia [ 3 ]. Children in the first months of life, particularly those with preterm birth, underlying chronic lung disease (CLD), congenital heart disease (CHD), neuromuscular disease, airway malformations or impaired cellular immunity are at risk for severe disease [ 4 - 6 ].Local epidemiological studies are important when considering new strategies for the prevention of RSV bronchiolitis. An example of such a strategy is the monthly injection of the humanised murine monoclonal antibody palivizumab to certain high-risk groups, which in many studies has shown a significant reduction in the incidence of hospitalisation for RSV bronchiolitis [ 7 ].The aims of the present study are to identify risk groups, outcome and incidences of hospitalisation for RSV bronchiolitis in Norwegian children under two years of age, and to compare these results with other studies.Methods Background population Akershus is a large suburban and countryside region located around the capital of Oslo with approximately 10% of the total population in Norway. The Paediatric Department at Akershus University Hospital (Ahus) is the only hospital for children under 15 years of age living in the 20 northern, southern and eastern communities of the region and serves all kinds of hospital services, except for paediatric surgery. Deliveries have been rather stable at approximately 3.500 per year in the service area during the last decade. Due to a local agreement infants from approximately 500 unselected term deliveries from our service area are born at Rikshospitalet in Oslo each year, but these infants are admitted to our hospital in case of disease during childhood. The number of children under two years of age living in the service area at any time during the follow-up period were identified from official Norwegian Population Statistics  . The number of children under two years of age born before completed 37 weeks of gestation in the service area during the study period and still alive after the neonatal period was identified from the Norwegian Medical Birth Registry  . All children with trisomy 21 have a program for follow-up from birth in our hospital and the number of children under two years of age with trisomy 21 was therefore identified from in-patient and out-patient hospital records. The number of children in the general population with a CHD was not known in the present study and hospitalisation incidences could therefore not be estimated. Overall mortality after the neonatal period as well as the number of children moving out of the service area during the study period were low and have not been corrected for. Study population The present study is a population-based retrospective survey on children under two years of age admitted to the hospital with a diagnosis of bronchiolitis (ICD 9 & 10) during the period February 5 th  1993 to January 31 th  2000 and diagnosed as positive with RSV in nasopharyngeal aspirate (NPA). RSV was diagnosed by an enzyme-linked immunosorbent assay (ELISA) for RSV (Abbott TestPack RSV). The test is simple and easy to perform giving a result within 30 minutes. The sensitivity, specificity, positive and negative predictive value is stated to be 74–92%, 86–100%, 81% and 93% respectively [ 8 , 9 ]. 764 children who fulfilled the diagnostic criteria were identified. A total of 77 children belonged to one or more well known high-risk groups. Of these, 58 children had been born before completed 37 weeks of gestation, some with additional diseases such as CLD (12 children) and CHD (four children). 12 children had a diagnosis of CHD as the only additional disease and seven children had trisomy 21, four of these also diagnosed with CHD. These 77 children were regarded as a high-risk group for subgroup analysis. No children with other well-known risk-factors (neuromuscular disease, airway malformations, impaired cellular immunity or others) were identified. The remaining 687 children were born at term and healthy and regarded as a low-risk group for subgroup analysis. Statistics Data were analysed with the statistical programme SPSS, version 11. For analysis of gender we used the Chi square test and for comparisons between low-risk and high-risk groups we used the independent sample T-test. A p-value <0.05 was used as limit for statistical significance.
Methods
Background population Akershus is a large suburban and countryside region located around the capital of Oslo with approximately 10% of the total population in Norway. The Paediatric Department at Akershus University Hospital (Ahus) is the only hospital for children under 15 years of age living in the 20 northern, southern and eastern communities of the region and serves all kinds of hospital services, except for paediatric surgery. Deliveries have been rather stable at approximately 3.500 per year in the service area during the last decade. Due to a local agreement infants from approximately 500 unselected term deliveries from our service area are born at Rikshospitalet in Oslo each year, but these infants are admitted to our hospital in case of disease during childhood. The number of children under two years of age living in the service area at any time during the follow-up period were identified from official Norwegian Population Statistics  . The number of children under two years of age born before completed 37 weeks of gestation in the service area during the study period and still alive after the neonatal period was identified from the Norwegian Medical Birth Registry  . All children with trisomy 21 have a program for follow-up from birth in our hospital and the number of children under two years of age with trisomy 21 was therefore identified from in-patient and out-patient hospital records. The number of children in the general population with a CHD was not known in the present study and hospitalisation incidences could therefore not be estimated. Overall mortality after the neonatal period as well as the number of children moving out of the service area during the study period were low and have not been corrected for.
Background population
Akershus is a large suburban and countryside region located around the capital of Oslo with approximately 10% of the total population in Norway. The Paediatric Department at Akershus University Hospital (Ahus) is the only hospital for children under 15 years of age living in the 20 northern, southern and eastern communities of the region and serves all kinds of hospital services, except for paediatric surgery. Deliveries have been rather stable at approximately 3.500 per year in the service area during the last decade. Due to a local agreement infants from approximately 500 unselected term deliveries from our service area are born at Rikshospitalet in Oslo each year, but these infants are admitted to our hospital in case of disease during childhood. The number of children under two years of age living in the service area at any time during the follow-up period were identified from official Norwegian Population Statistics  . The number of children under two years of age born before completed 37 weeks of gestation in the service area during the study period and still alive after the neonatal period was identified from the Norwegian Medical Birth Registry  . All children with trisomy 21 have a program for follow-up from birth in our hospital and the number of children under two years of age with trisomy 21 was therefore identified from in-patient and out-patient hospital records. The number of children in the general population with a CHD was not known in the present study and hospitalisation incidences could therefore not be estimated. Overall mortality after the neonatal period as well as the number of children moving out of the service area during the study period were low and have not been corrected for.Study population The present study is a population-based retrospective survey on children under two years of age admitted to the hospital with a diagnosis of bronchiolitis (ICD 9 & 10) during the period February 5 th  1993 to January 31 th  2000 and diagnosed as positive with RSV in nasopharyngeal aspirate (NPA). RSV was diagnosed by an enzyme-linked immunosorbent assay (ELISA) for RSV (Abbott TestPack RSV). The test is simple and easy to perform giving a result within 30 minutes. The sensitivity, specificity, positive and negative predictive value is stated to be 74–92%, 86–100%, 81% and 93% respectively [ 8 , 9 ]. 764 children who fulfilled the diagnostic criteria were identified. A total of 77 children belonged to one or more well known high-risk groups. Of these, 58 children had been born before completed 37 weeks of gestation, some with additional diseases such as CLD (12 children) and CHD (four children). 12 children had a diagnosis of CHD as the only additional disease and seven children had trisomy 21, four of these also diagnosed with CHD. These 77 children were regarded as a high-risk group for subgroup analysis. No children with other well-known risk-factors (neuromuscular disease, airway malformations, impaired cellular immunity or others) were identified. The remaining 687 children were born at term and healthy and regarded as a low-risk group for subgroup analysis.
Study population
The present study is a population-based retrospective survey on children under two years of age admitted to the hospital with a diagnosis of bronchiolitis (ICD 9 & 10) during the period February 5 th  1993 to January 31 th  2000 and diagnosed as positive with RSV in nasopharyngeal aspirate (NPA). RSV was diagnosed by an enzyme-linked immunosorbent assay (ELISA) for RSV (Abbott TestPack RSV). The test is simple and easy to perform giving a result within 30 minutes. The sensitivity, specificity, positive and negative predictive value is stated to be 74–92%, 86–100%, 81% and 93% respectively [ 8 , 9 ].764 children who fulfilled the diagnostic criteria were identified. A total of 77 children belonged to one or more well known high-risk groups. Of these, 58 children had been born before completed 37 weeks of gestation, some with additional diseases such as CLD (12 children) and CHD (four children). 12 children had a diagnosis of CHD as the only additional disease and seven children had trisomy 21, four of these also diagnosed with CHD. These 77 children were regarded as a high-risk group for subgroup analysis. No children with other well-known risk-factors (neuromuscular disease, airway malformations, impaired cellular immunity or others) were identified. The remaining 687 children were born at term and healthy and regarded as a low-risk group for subgroup analysis.Statistics Data were analysed with the statistical programme SPSS, version 11. For analysis of gender we used the Chi square test and for comparisons between low-risk and high-risk groups we used the independent sample T-test. A p-value <0.05 was used as limit for statistical significance.
Statistics
Data were analysed with the statistical programme SPSS, version 11. For analysis of gender we used the Chi square test and for comparisons between low-risk and high-risk groups we used the independent sample T-test. A p-value <0.05 was used as limit for statistical significance.Results Epidemiology During this seven years follow-up period 764 children (822 admissions) were hospitalised for RSV bronchiolitis. 93% (707 children) had only one hospitalisation and 7% (57 children) had two or more hospitalisations for RSV bronchiolitis during the first two years of life. The highest number of admissions was recorded during the winter months December-April, but cases were also found in late spring and early autumn. Both the number of admissions and peak-time varied between seasons but with no specific pattern of variability (figure  1 ). A significant male predominance was observed, with 517 (63%) of all admissions being boys (P < 0.001) Figure 1 Seasonal variations by month and year in hospitalisations for RSV bronchiolitis in children under two years of age in Akershus, Norway, February 1993 to January 2000 The majority of children (75% of all admissions) were hospitalised within the first year of life with children less than six months old being responsible for 45% of all admissions. Median age at hospitalisation was 6 months (range 0–23 months) and a median length of stay of 4 days (range 1–41 days) was observed (table  1 ). Table 1 Age on admission and length of stay in children under two years of age hospitalised for RSV bronchiolitis in Akershus, Norway, February 1993 through January 2000 Risk groups Children (no.) Adm (no.) Median age a P-value (age) Median stay b P-value (stay) All children 764 822 6.0 4.0 Premature c 58 64 8.0 NS e 8.0 <0.001 e Corrected age d 58 64 5.4 NS e Trisomy 21 7 8 9.0 NS e 7.5 <0.001 e CHD* 12 13 7.0 NS e 6.0 NS e All low-risk 687 737 6.0 4.0 All high-risk 77 85 8.0 0.045 e 8.0 <0.001 e Corrected age d 77 85 5.5 NS e a Age in months b Days including admission day c Born at <37 weeks of gestational age d Corrected for weeks of prematurity e Versus low-risk children During the study period nine children (1.2%) developed severe respiratory distress and needed mechanical ventilation and two of them (0.3%) died. Four of these children had no known risk-factors for severe disease. However, the two children who died were both considered as high-risk patients, one of them was diagnosed with CHD and the other with trisomy 21 as well as CHD. Of all RSV hospitalisations during the first two years of life 58 children (64 admissions) were born before completed 37 weeks of gestation with a median gestational age of 30 weeks (range 24–36 weeks). A significant male predominance of 68% was also observed in this group (P = 0.006). Median age on admission, after postnatal age, was 8 months (range 0–23 months). When corrected for prematurity (corrected age), median age on admission was 5.4 months (range -2.5–21.3 months). Also, a median length of hospitalisation of 8 days (range 2–27 days) was observed (table  1 ). Two of the preterm children (3.4%) needed mechanical ventilation and two were treated with inhaled nebulized ribavirin [ 10 ]. No preterm children in this study died from RSV bronchiolitis. Seven children (eight admissions) diagnosed with trisomy 21 were hospitalised for RSV bronchiolitis during this follow-up study. Four of them also had a CHD. Two children needed mechanical ventilation during hospitalisation and one of them subsequently died. A median age on admission of 9.0 months (range 1–20 months) and a median stay of 7.5 days (range 2–34 days) were recorded for children in this particular risk group (table  1 ). A total of 20 children had a CHD, four combined with prematurity, four combined with trisomy 21, and 12 with no other additional risk-factor for severe disease. In the group of children with only CHD as risk factor median age on admission was 7.0 months (range 0–23 months) and median stay of 6.0 days (range 2–14 days) was observed (table  1 ). One child needed mechanical ventilation and subsequently died. Hospitalisation incidences Overall hospitalisation incidences were calculated from the known number by official Norwegian statistics of 58.179 children under two years of age living in the service area for the whole study period as well as for each year as given in table  2 . As shown, the overall incidences for the whole study population was 21.7 admissions per 1.000 children under one year of age, 6.8 admissions per 1.000 children 1–2 years of age and 14.1 admissions per 1.000 children under two years of age, with some year to year variation (table  2 ). Table 2 Mean annual hospitalisation incidences per 1.000 children by age and risk-groups in children under two years of age with RSV bronchiolitis in Akershus, Norway, 1993–2000 Children <1 year Children 1–2 years Children 0–2 years Year a Pop d Adm Inc Pop d Adm Inc Pop d Adm Inc All 93–94 4.027 70 17.4 4.195 16 3.8 8.222 86 10.5 All 94–95 3.999 53 13.3 4.192 25 6.0 8.191 78 9.5 All 95–96 3.994 112 28.0 4.132 30 7.3 8.126 142 17.5 All 96–97 4.055 80 19.7 4.143 22 5.3 8.198 102 12.4 All 97–98 4.154 115 27.7 4.242 57 13.4 8.396 172 20.5 All 98–99 4.178 70 16.8 4.350 15 3.4 8.528 85 10.0 All 99–00 4.127 120 29.1 4.391 37 8.4 8.518 157 18.4 All 93–2000 28.534 620 21.7 29.645 202 6.8 58.179 822 14.1 Premature b  93–2000 1.999 c 47 23.5 1.955 c 17 8.7 3.954 c 64 16.2 Trisomy 21 93–2000 52 8 153.8 a Study period February 5th, 1993 to January 31th, 2000 b Born at <37 weeks of gestational age c Number of children born at <37 weeks of gestation and corrected for a mean infant mortality rate of 2.4% during the neonatal period d Background population for selected risk groups Data from the Norwegian Birth Registry identified that 1.999 infants born in the service area before completed 37 weeks of gestation were alive and below 1 year of age, and 1.955 infants were alive and between 1–2 years of age during the study period. As shown, the corresponding hospitalisation incidences for preterm children were 23.5, 8.7 and 16.2 per 1.000 children under one year of age, 1–2 years of age and <2 years of age respectively (table  2 ). From the hospital records 52 children under two years of age were diagnosed with trisomy 21 during the follow-up period. Among these seven children (eight admissions) were hospitalised for RSV bronchiolitis, giving a hospitalisation incidence under two years of 153.8 per 1.000 children.
Results
Epidemiology During this seven years follow-up period 764 children (822 admissions) were hospitalised for RSV bronchiolitis. 93% (707 children) had only one hospitalisation and 7% (57 children) had two or more hospitalisations for RSV bronchiolitis during the first two years of life. The highest number of admissions was recorded during the winter months December-April, but cases were also found in late spring and early autumn. Both the number of admissions and peak-time varied between seasons but with no specific pattern of variability (figure  1 ). A significant male predominance was observed, with 517 (63%) of all admissions being boys (P < 0.001) Figure 1 Seasonal variations by month and year in hospitalisations for RSV bronchiolitis in children under two years of age in Akershus, Norway, February 1993 to January 2000 The majority of children (75% of all admissions) were hospitalised within the first year of life with children less than six months old being responsible for 45% of all admissions. Median age at hospitalisation was 6 months (range 0–23 months) and a median length of stay of 4 days (range 1–41 days) was observed (table  1 ). Table 1 Age on admission and length of stay in children under two years of age hospitalised for RSV bronchiolitis in Akershus, Norway, February 1993 through January 2000 Risk groups Children (no.) Adm (no.) Median age a P-value (age) Median stay b P-value (stay) All children 764 822 6.0 4.0 Premature c 58 64 8.0 NS e 8.0 <0.001 e Corrected age d 58 64 5.4 NS e Trisomy 21 7 8 9.0 NS e 7.5 <0.001 e CHD* 12 13 7.0 NS e 6.0 NS e All low-risk 687 737 6.0 4.0 All high-risk 77 85 8.0 0.045 e 8.0 <0.001 e Corrected age d 77 85 5.5 NS e a Age in months b Days including admission day c Born at <37 weeks of gestational age d Corrected for weeks of prematurity e Versus low-risk children During the study period nine children (1.2%) developed severe respiratory distress and needed mechanical ventilation and two of them (0.3%) died. Four of these children had no known risk-factors for severe disease. However, the two children who died were both considered as high-risk patients, one of them was diagnosed with CHD and the other with trisomy 21 as well as CHD. Of all RSV hospitalisations during the first two years of life 58 children (64 admissions) were born before completed 37 weeks of gestation with a median gestational age of 30 weeks (range 24–36 weeks). A significant male predominance of 68% was also observed in this group (P = 0.006). Median age on admission, after postnatal age, was 8 months (range 0–23 months). When corrected for prematurity (corrected age), median age on admission was 5.4 months (range -2.5–21.3 months). Also, a median length of hospitalisation of 8 days (range 2–27 days) was observed (table  1 ). Two of the preterm children (3.4%) needed mechanical ventilation and two were treated with inhaled nebulized ribavirin [ 10 ]. No preterm children in this study died from RSV bronchiolitis. Seven children (eight admissions) diagnosed with trisomy 21 were hospitalised for RSV bronchiolitis during this follow-up study. Four of them also had a CHD. Two children needed mechanical ventilation during hospitalisation and one of them subsequently died. A median age on admission of 9.0 months (range 1–20 months) and a median stay of 7.5 days (range 2–34 days) were recorded for children in this particular risk group (table  1 ). A total of 20 children had a CHD, four combined with prematurity, four combined with trisomy 21, and 12 with no other additional risk-factor for severe disease. In the group of children with only CHD as risk factor median age on admission was 7.0 months (range 0–23 months) and median stay of 6.0 days (range 2–14 days) was observed (table  1 ). One child needed mechanical ventilation and subsequently died.
Epidemiology
During this seven years follow-up period 764 children (822 admissions) were hospitalised for RSV bronchiolitis. 93% (707 children) had only one hospitalisation and 7% (57 children) had two or more hospitalisations for RSV bronchiolitis during the first two years of life. The highest number of admissions was recorded during the winter months December-April, but cases were also found in late spring and early autumn. Both the number of admissions and peak-time varied between seasons but with no specific pattern of variability (figure  1 ). A significant male predominance was observed, with 517 (63%) of all admissions being boys (P < 0.001)The majority of children (75% of all admissions) were hospitalised within the first year of life with children less than six months old being responsible for 45% of all admissions. Median age at hospitalisation was 6 months (range 0–23 months) and a median length of stay of 4 days (range 1–41 days) was observed (table  1 ).Age on admission and length of stay in children under two years of age hospitalised for RSV bronchiolitis in Akershus, Norway, February 1993 through January 2000a Age in monthsb Days including admission dayc Born at <37 weeks of gestational aged Corrected for weeks of prematuritye Versus low-risk childrenDuring the study period nine children (1.2%) developed severe respiratory distress and needed mechanical ventilation and two of them (0.3%) died. Four of these children had no known risk-factors for severe disease. However, the two children who died were both considered as high-risk patients, one of them was diagnosed with CHD and the other with trisomy 21 as well as CHD.Of all RSV hospitalisations during the first two years of life 58 children (64 admissions) were born before completed 37 weeks of gestation with a median gestational age of 30 weeks (range 24–36 weeks). A significant male predominance of 68% was also observed in this group (P = 0.006). Median age on admission, after postnatal age, was 8 months (range 0–23 months). When corrected for prematurity (corrected age), median age on admission was 5.4 months (range -2.5–21.3 months). Also, a median length of hospitalisation of 8 days (range 2–27 days) was observed (table  1 ). Two of the preterm children (3.4%) needed mechanical ventilation and two were treated with inhaled nebulized ribavirin [ 10 ]. No preterm children in this study died from RSV bronchiolitis.Seven children (eight admissions) diagnosed with trisomy 21 were hospitalised for RSV bronchiolitis during this follow-up study. Four of them also had a CHD. Two children needed mechanical ventilation during hospitalisation and one of them subsequently died.A median age on admission of 9.0 months (range 1–20 months) and a median stay of 7.5 days (range 2–34 days) were recorded for children in this particular risk group (table  1 ).A total of 20 children had a CHD, four combined with prematurity, four combined with trisomy 21, and 12 with no other additional risk-factor for severe disease. In the group of children with only CHD as risk factor median age on admission was 7.0 months (range 0–23 months) and median stay of 6.0 days (range 2–14 days) was observed (table  1 ). One child needed mechanical ventilation and subsequently died.Hospitalisation incidences Overall hospitalisation incidences were calculated from the known number by official Norwegian statistics of 58.179 children under two years of age living in the service area for the whole study period as well as for each year as given in table  2 . As shown, the overall incidences for the whole study population was 21.7 admissions per 1.000 children under one year of age, 6.8 admissions per 1.000 children 1–2 years of age and 14.1 admissions per 1.000 children under two years of age, with some year to year variation (table  2 ). Table 2 Mean annual hospitalisation incidences per 1.000 children by age and risk-groups in children under two years of age with RSV bronchiolitis in Akershus, Norway, 1993–2000 Children <1 year Children 1–2 years Children 0–2 years Year a Pop d Adm Inc Pop d Adm Inc Pop d Adm Inc All 93–94 4.027 70 17.4 4.195 16 3.8 8.222 86 10.5 All 94–95 3.999 53 13.3 4.192 25 6.0 8.191 78 9.5 All 95–96 3.994 112 28.0 4.132 30 7.3 8.126 142 17.5 All 96–97 4.055 80 19.7 4.143 22 5.3 8.198 102 12.4 All 97–98 4.154 115 27.7 4.242 57 13.4 8.396 172 20.5 All 98–99 4.178 70 16.8 4.350 15 3.4 8.528 85 10.0 All 99–00 4.127 120 29.1 4.391 37 8.4 8.518 157 18.4 All 93–2000 28.534 620 21.7 29.645 202 6.8 58.179 822 14.1 Premature b  93–2000 1.999 c 47 23.5 1.955 c 17 8.7 3.954 c 64 16.2 Trisomy 21 93–2000 52 8 153.8 a Study period February 5th, 1993 to January 31th, 2000 b Born at <37 weeks of gestational age c Number of children born at <37 weeks of gestation and corrected for a mean infant mortality rate of 2.4% during the neonatal period d Background population for selected risk groups Data from the Norwegian Birth Registry identified that 1.999 infants born in the service area before completed 37 weeks of gestation were alive and below 1 year of age, and 1.955 infants were alive and between 1–2 years of age during the study period. As shown, the corresponding hospitalisation incidences for preterm children were 23.5, 8.7 and 16.2 per 1.000 children under one year of age, 1–2 years of age and <2 years of age respectively (table  2 ). From the hospital records 52 children under two years of age were diagnosed with trisomy 21 during the follow-up period. Among these seven children (eight admissions) were hospitalised for RSV bronchiolitis, giving a hospitalisation incidence under two years of 153.8 per 1.000 children.
Hospitalisation incidences
Overall hospitalisation incidences were calculated from the known number by official Norwegian statistics of 58.179 children under two years of age living in the service area for the whole study period as well as for each year as given in table  2 . As shown, the overall incidences for the whole study population was 21.7 admissions per 1.000 children under one year of age, 6.8 admissions per 1.000 children 1–2 years of age and 14.1 admissions per 1.000 children under two years of age, with some year to year variation (table  2 ).Mean annual hospitalisation incidences per 1.000 children by age and risk-groups in children under two years of age with RSV bronchiolitis in Akershus, Norway, 1993–2000a Study period February 5th, 1993 to January 31th, 2000b Born at <37 weeks of gestational agec Number of children born at <37 weeks of gestation and corrected for a mean infant mortality rate of 2.4% during the neonatal periodd Background population for selected risk groupsData from the Norwegian Birth Registry identified that 1.999 infants born in the service area before completed 37 weeks of gestation were alive and below 1 year of age, and 1.955 infants were alive and between 1–2 years of age during the study period. As shown, the corresponding hospitalisation incidences for preterm children were 23.5, 8.7 and 16.2 per 1.000 children under one year of age, 1–2 years of age and <2 years of age respectively (table  2 ).From the hospital records 52 children under two years of age were diagnosed with trisomy 21 during the follow-up period. Among these seven children (eight admissions) were hospitalised for RSV bronchiolitis, giving a hospitalisation incidence under two years of 153.8 per 1.000 children.Discussion In this retrospective study, as in many other comparable epidemiological studies, we found that RSV bronchiolitis appears as an annual winter epidemic with relatively high hospitalisation incidences, predominance of boys, young age, short length of stay, few complications and low mortality [ 1 ]. The rather long study period makes the results less vulnerable for yearly variations in magnitude of the annual epidemic and in virulence of the microbe. By using NPA for microbiological diagnosis in all children with symptoms of a lower respiratory tract infection like tachypnoe, wheezing or apnoic spells we believe that the great majority of cases are included. However, the enzyme-linked immunosorbent assay used in the present study could give both false positive and false negative results [ 9 , 11 ]. Several international studies have shown an increased incidence of RSV-related hospitalisations over the last two decades. A Norwegian study for the period 1972–78 showed a mean hospitalisation incidence of 9.5 per 1.000 children for children under one year of age compared to our much higher incidence of 21.7 per 1.000 children for the period 1993–2000 in the same age group [ 12 ]. Also, for children 1–2 years old the incidence in our study was much higher. A large study from USA for the period 1980–96 showed an increased incidence rising from 12.9 per 1.000 children in 1980 to 31.2 per 1.000 children in 1996 for children under one year of age [ 13 ]. However, comparing historical data is difficult, and for the period 1993–2000 we observed no increase in the mean annual hospitalisation incidences of RSV bronchiolitis. Factors such as increased microbial virulence, increased day-care attendance, improved microbiological diagnosis and more precise ICD-coding might all be important factors to explain these historical differences. Number of hospitalisations are also related to the severity of the clinical symptoms. Thus, changes in hospital admission policies over time as well as changes in availability of hospital services might also influence hospitalisation incidences. A recent study from USA for the period 1997–99 showed that RSV bronchiolitis was the leading cause of hospital admissions of infants younger than one year of age with an associated hospitalisation incidence of 25.2 per 1.000 infants. This is in accordance with our findings [ 14 ]. Our study showed that only 1.2% of the children in the study population needed mechanical ventilation. This result is lower than in many other studies [ 15 ]. On the other hand, when children were in need of mechanical ventilation, the risk of severe outcome was in our study very high, with two out of nine mechanically ventilated children subsequently dying. A Danish study showed that only 0.6% of their children needed mechanical ventilation, however, 20% of all children with severe respiratory failure were given ventilatory support with nasal continuous positive airway pressure (N-CPAP) and some infants possibly avoided mechanical ventilation for that reason [ 16 ]. More controlled studies on different treatment strategies are very much needed, especially when considering how to avoid respiratory failure and mechanical ventilation [ 17 , 18 ]. Another observation in our study was that among the mechanically ventilated children 44% had no underlying risk-factors for severe disease. Further research into why some otherwise healthy children have a severe course of RSV bronchiolitis is therefore very important, not least when trying to define risk groups even better [ 19 ]. Research for a better understanding of the host immune response to RSV bronchiolitis the later years might be one important piece in this puzzle [ 20 - 22 ]. For the population at large our study showed, as in other studies, a low mortality. A total of two children (0.3%) died, one had trisomy 21 with an atrioventricular septum defect (AVSD) and the other had an underlying CHD. The number of high-risk children in our study is relatively small and the results should be interpreted with caution. The mean hospitalisation incidences for preterm children were lower than in many other studies, even though 20% of the preterm children suffered from CLD as well and had a low median weeks of gestational age (wGA) of 30. One possible explanation is that the population data for this particular risk group was not corrected for migration out of our service area and mortality after the neonatal period. However, it is unlikely that such corrections would significantly influence the results. Another explanation for the low incidence among preterm infants might be that most of them are hospitalised up to almost 40 wGA and are discharged only when the child has reached a weight of more than 2.000 g and have normal oral feedings. This means that they are more or less completely protected from RSV for a long period after birth. Our routines with focus on good parental information on how to protect the newborn infant from RSV after discharge from hospital might also be important. Median length of hospitalisation for preterm children was in our study significantly longer when compared to the low-risk children and is in accordance with results from many other larger studies. Children with CHD are a wellknown risk-group for severe RSV disease. Children with trisomy 21 with or without a concomitant CHD should also be considered a high-risk group. Our study showed a mean hospitalisation incidence per 1.000 children under two years of age with trisomy 21 more than 11 times the whole study population. Even more important, out of a total of seven children with trisomy 21, two needed mechanical ventilation and one child died. We would, however, recommend a larger study to meet these observations for children with trisomy 21. One of the findings in our study was the large proportion of otherwise healthy children needing mechanical ventilation. Our numbers are small, but when new policies to prevent severe RSV bronchiolitis are discussed this aspect should also be considered. The research for improved identification of risk-groups and development of an effective vaccine for immunization should therefore be given high priority.
Discussion
In this retrospective study, as in many other comparable epidemiological studies, we found that RSV bronchiolitis appears as an annual winter epidemic with relatively high hospitalisation incidences, predominance of boys, young age, short length of stay, few complications and low mortality [ 1 ]. The rather long study period makes the results less vulnerable for yearly variations in magnitude of the annual epidemic and in virulence of the microbe. By using NPA for microbiological diagnosis in all children with symptoms of a lower respiratory tract infection like tachypnoe, wheezing or apnoic spells we believe that the great majority of cases are included. However, the enzyme-linked immunosorbent assay used in the present study could give both false positive and false negative results [ 9 , 11 ].Several international studies have shown an increased incidence of RSV-related hospitalisations over the last two decades. A Norwegian study for the period 1972–78 showed a mean hospitalisation incidence of 9.5 per 1.000 children for children under one year of age compared to our much higher incidence of 21.7 per 1.000 children for the period 1993–2000 in the same age group [ 12 ]. Also, for children 1–2 years old the incidence in our study was much higher. A large study from USA for the period 1980–96 showed an increased incidence rising from 12.9 per 1.000 children in 1980 to 31.2 per 1.000 children in 1996 for children under one year of age [ 13 ]. However, comparing historical data is difficult, and for the period 1993–2000 we observed no increase in the mean annual hospitalisation incidences of RSV bronchiolitis. Factors such as increased microbial virulence, increased day-care attendance, improved microbiological diagnosis and more precise ICD-coding might all be important factors to explain these historical differences. Number of hospitalisations are also related to the severity of the clinical symptoms. Thus, changes in hospital admission policies over time as well as changes in availability of hospital services might also influence hospitalisation incidences. A recent study from USA for the period 1997–99 showed that RSV bronchiolitis was the leading cause of hospital admissions of infants younger than one year of age with an associated hospitalisation incidence of 25.2 per 1.000 infants. This is in accordance with our findings [ 14 ].Our study showed that only 1.2% of the children in the study population needed mechanical ventilation. This result is lower than in many other studies [ 15 ]. On the other hand, when children were in need of mechanical ventilation, the risk of severe outcome was in our study very high, with two out of nine mechanically ventilated children subsequently dying. A Danish study showed that only 0.6% of their children needed mechanical ventilation, however, 20% of all children with severe respiratory failure were given ventilatory support with nasal continuous positive airway pressure (N-CPAP) and some infants possibly avoided mechanical ventilation for that reason [ 16 ]. More controlled studies on different treatment strategies are very much needed, especially when considering how to avoid respiratory failure and mechanical ventilation [ 17 , 18 ].Another observation in our study was that among the mechanically ventilated children 44% had no underlying risk-factors for severe disease. Further research into why some otherwise healthy children have a severe course of RSV bronchiolitis is therefore very important, not least when trying to define risk groups even better [ 19 ]. Research for a better understanding of the host immune response to RSV bronchiolitis the later years might be one important piece in this puzzle [ 20 - 22 ].For the population at large our study showed, as in other studies, a low mortality. A total of two children (0.3%) died, one had trisomy 21 with an atrioventricular septum defect (AVSD) and the other had an underlying CHD.The number of high-risk children in our study is relatively small and the results should be interpreted with caution. The mean hospitalisation incidences for preterm children were lower than in many other studies, even though 20% of the preterm children suffered from CLD as well and had a low median weeks of gestational age (wGA) of 30. One possible explanation is that the population data for this particular risk group was not corrected for migration out of our service area and mortality after the neonatal period. However, it is unlikely that such corrections would significantly influence the results. Another explanation for the low incidence among preterm infants might be that most of them are hospitalised up to almost 40 wGA and are discharged only when the child has reached a weight of more than 2.000 g and have normal oral feedings. This means that they are more or less completely protected from RSV for a long period after birth. Our routines with focus on good parental information on how to protect the newborn infant from RSV after discharge from hospital might also be important. Median length of hospitalisation for preterm children was in our study significantly longer when compared to the low-risk children and is in accordance with results from many other larger studies.Children with CHD are a wellknown risk-group for severe RSV disease. Children with trisomy 21 with or without a concomitant CHD should also be considered a high-risk group. Our study showed a mean hospitalisation incidence per 1.000 children under two years of age with trisomy 21 more than 11 times the whole study population. Even more important, out of a total of seven children with trisomy 21, two needed mechanical ventilation and one child died. We would, however, recommend a larger study to meet these observations for children with trisomy 21.One of the findings in our study was the large proportion of otherwise healthy children needing mechanical ventilation. Our numbers are small, but when new policies to prevent severe RSV bronchiolitis are discussed this aspect should also be considered. The research for improved identification of risk-groups and development of an effective vaccine for immunization should therefore be given high priority.Conclusions While the overall hospitalisation incidences and outcome of RSV bronchiolitis were in agreement with other studies, hospitalisation incidences for preterm children were lower than in many other studies. Age on admission for preterm children, when corrected for prematurity, was comparable to low-risk children. Length of hospitalisation and morbidity was high in both preterm children, children with a congenital heart disease and in children with trisomy 21, the last group being at particular high risk for severe disease.
Conclusions
While the overall hospitalisation incidences and outcome of RSV bronchiolitis were in agreement with other studies, hospitalisation incidences for preterm children were lower than in many other studies. Age on admission for preterm children, when corrected for prematurity, was comparable to low-risk children. Length of hospitalisation and morbidity was high in both preterm children, children with a congenital heart disease and in children with trisomy 21, the last group being at particular high risk for severe disease.Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Authors' contributions Our individual contributions to the study have been as following: HOF had primary responsibility for protocol development, outcome assessment, data acquisition and analysis and writing of the manuscript. TF participated in the development of the protocol and analytic framework of the study, and contributed to the writing of the manuscript. DB participated in the data analyses and the writing of the manuscript. All authors read and approved the final manuscript.
Authors' contributions
Our individual contributions to the study have been as following: HOF had primary responsibility for protocol development, outcome assessment, data acquisition and analysis and writing of the manuscript. TF participated in the development of the protocol and analytic framework of the study, and contributed to the writing of the manuscript. DB participated in the data analyses and the writing of the manuscript. All authors read and approved the final manuscript.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here:


Background One of every 6 United Status birth certificates contains no information on fathers. There might be important differences in the pregnancy outcomes between mothers with versus those without partner information. The object of this study was to assess whether and to what extent outcomes in pregnant women who did not have partner information differ from those who had.
Background
One of every 6 United Status birth certificates contains no information on fathers. There might be important differences in the pregnancy outcomes between mothers with versus those without partner information. The object of this study was to assess whether and to what extent outcomes in pregnant women who did not have partner information differ from those who had.Methods We carried out a population-based retrospective cohort study based on the registry data in the United States for the period of 1995–1997, which was a matched multiple birth file (only twins were included in the current analysis). We divided the study subjects into three groups according to the availability of partner information: available, partly missing, and totally missing. We compared the distribution of maternal characteristics, maternal morbidity, labor and delivery complications, obstetric interventions, preterm birth, fetal growth restriction, low birth weight, congenital anomalies, fetal death, neonatal death, post-neonatal death, and neonatal morbidity among three study groups.
Methods
We carried out a population-based retrospective cohort study based on the registry data in the United States for the period of 1995–1997, which was a matched multiple birth file (only twins were included in the current analysis). We divided the study subjects into three groups according to the availability of partner information: available, partly missing, and totally missing. We compared the distribution of maternal characteristics, maternal morbidity, labor and delivery complications, obstetric interventions, preterm birth, fetal growth restriction, low birth weight, congenital anomalies, fetal death, neonatal death, post-neonatal death, and neonatal morbidity among three study groups.Results There were 304466 twins included in our study. Mothers whose partner's information was partly missing and (especially) totally missing tended to be younger, of black race, unmarried, with less education, smoking cigarette during pregnancy, and with inadequate prenatal care. The rates of preterm birth, fetal growth restriction, low birth weight, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality were significantly increased in mothers whose partner's information was partly or (especially) totally missing.
Results
There were 304466 twins included in our study. Mothers whose partner's information was partly missing and (especially) totally missing tended to be younger, of black race, unmarried, with less education, smoking cigarette during pregnancy, and with inadequate prenatal care. The rates of preterm birth, fetal growth restriction, low birth weight, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality were significantly increased in mothers whose partner's information was partly or (especially) totally missing.Conclusions Mothers whose partner's information was partly and (especially) totally missing are at higher risk of adverse pregnant outcomes, and clinicians and public health workers should be alerted to this important social factor.
Conclusions
Mothers whose partner's information was partly and (especially) totally missing are at higher risk of adverse pregnant outcomes, and clinicians and public health workers should be alerted to this important social factor.

Background Pregnancy outcomes are not only important measures of health status of the mothers and infants, but are important measures of socioeconomic development in a society. For example, infant mortality has been considered the single most comprehensive measure of the health and wealth in a society [ 1 , 2 ]. Both maternal and paternal characteristics, such as age, race, marital status, education, and cigarette smoking [ 3 - 5 ], are important determinants of pregnancy outcomes. However, partner information is often missing in routine vital statistics data. For example, about 17% of United Status birth certificates contain no paternal information and more than 40% of the babies born to adolescent women have no information on the paternal age in the birth certificate [ 6 ]. There might be important differences in the pregnancy outcomes between mothers with versus those without paternal information. Detailed description of these differences may benefit the clinicians and public health workers who serve high risk pregnant women. We conducted extensive search of the literature and have not been able to locate a single study on this issue. Epidemiologic studies based on a data with missing information on exposure, outcomes, and potential confounders can bias the study results, unless the missing occurred randomly [ 7 ]. Previous studies on missing data [ 8 - 11 ] have focused on the impact of missing information on study results and the methods to treat the missing variables. The main objective of the current study is to describe the distribution of maternal characteristics and pregnancy outcomes in mothers whose partner's information was partly or totally missing. We also attempted to assess the potential source of missing partner information and its impact on outcomes, by exploring the reasons of differences between partly missing versus totally missing. We used a large twin registry data in the United States to examine this issue. This data has been used in several peer-reviewed studies [ 12 - 14 ]. The reasons of using data on twins were two folds. First, adverse pregnancy outcomes are more common in twins than in singletons. Therefore, it is easier to detect the difference in adverse pregnancy outcomes in twins than in singletons. Second, because linkage for multiple births requires an examination and processing of the recorded variables, it will increase the validity of the administrative data.
Background
Pregnancy outcomes are not only important measures of health status of the mothers and infants, but are important measures of socioeconomic development in a society. For example, infant mortality has been considered the single most comprehensive measure of the health and wealth in a society [ 1 , 2 ]. Both maternal and paternal characteristics, such as age, race, marital status, education, and cigarette smoking [ 3 - 5 ], are important determinants of pregnancy outcomes. However, partner information is often missing in routine vital statistics data. For example, about 17% of United Status birth certificates contain no paternal information and more than 40% of the babies born to adolescent women have no information on the paternal age in the birth certificate [ 6 ]. There might be important differences in the pregnancy outcomes between mothers with versus those without paternal information. Detailed description of these differences may benefit the clinicians and public health workers who serve high risk pregnant women. We conducted extensive search of the literature and have not been able to locate a single study on this issue.Epidemiologic studies based on a data with missing information on exposure, outcomes, and potential confounders can bias the study results, unless the missing occurred randomly [ 7 ]. Previous studies on missing data [ 8 - 11 ] have focused on the impact of missing information on study results and the methods to treat the missing variables. The main objective of the current study is to describe the distribution of maternal characteristics and pregnancy outcomes in mothers whose partner's information was partly or totally missing. We also attempted to assess the potential source of missing partner information and its impact on outcomes, by exploring the reasons of differences between partly missing versus totally missing.We used a large twin registry data in the United States to examine this issue. This data has been used in several peer-reviewed studies [ 12 - 14 ]. The reasons of using data on twins were two folds. First, adverse pregnancy outcomes are more common in twins than in singletons. Therefore, it is easier to detect the difference in adverse pregnancy outcomes in twins than in singletons. Second, because linkage for multiple births requires an examination and processing of the recorded variables, it will increase the validity of the administrative data.Methods We used the matched multiple birth file created by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention [ 15 ]. The multiple birth file in 1995–1997 in the United States were linked infant deaths information according to uniform coding specifications, which have undergone vigorous editing and reviewing during the process of the record linkage. Sets of multiples in the 1995–1997 birth file were matched by plurality, state and county of occurrence of delivery, mother's date of birth, date of last menstrual period (LMP), number of prenatal visits, level of education, weight gain during pregnancy, and date of delivery. The matching was successful for 98% of the multiple sets [ 15 ]. Available study variables in the database include socio-demographic information of the parents, maternal life-style factors such as smoking during pregnancy, obstetric history, complications of the pregnancy, labor and delivery, birth weight, gestational age, and other infant outcome variables. Only complete twin sets were included in the current study, with twin set as the unit of analysis in comparison of maternal characteristics and outcomes, and randomly selected one twin in each twin sets as the unit of analysis for fetal and neonatal outcomes. There were two variables for the partners in the data: partner's age and race. The eligible study subjects were divided into three groups according to the availability of the two variables of the partners: both available (group 1), one missing (group 2), and both missing (group 3). We first described the distribution of the maternal characteristics (age, race, marital status, education, cigarette smoking, place of birth, and prenatal care service) of the three study groups. The prenatal care service was defined as adequate, intermediate, and inadequate according to the time of prenatal care visit initiation and the number of prenatal visit using the method described by Kessner et al [ 16 ]. If the first prenatal care initiated at third trimester, or the times of prenatal visit less than 5 in 34 gestation weeks, 4 in 32–33 weeks, 3 in 30–31 weeks, or 2 in 22–29 weeks, the prenatal care service was defined as inadequate; if the first prenatal care initiated at first trimester, and the times of prenatal visit more than 8 in 36 gestation weeks, 7 in 34–35 weeks, 6 in 32–33 weeks, 5 in 30–31 weeks, 4 in 26–29 weeks, 3 in 22–25 weeks, or 2 in 18–21 weeks, the prenatal care service was defined as adequate; and the remainders were defined as intermediate. We compared the rates of maternal medical complications (anemia, cardiac disease, acute or chronic lung disease, diabetes, genital herpes, hydramnios, oligohydramnios, hemoglobinopathy, chronic hypertension, pregnancy-associated hypertension, eclampsia, incompetent cervix, renal disease, RH sensitization, and uterine bleeding), labor and delivery complications (febrile (any temperature reading >38°C), meconium, premature rupture of membrane (>12 hours), abruption placenta, plancenta previa, seizures during labor, precipitous labor (2 nd  stage <3 hours), prolonged labor (2 nd  stage >20 hours), dysfunctional labor, breech, malpresentation, cephalopelvic disproportion, cord prolapse, anesthetic complications, and fetal distress), and obstetric interventions (cesarean section, vacuum / forceps, and labor inductions) among the three study groups. Finally we compared the rates of fetal and neonatal mortality and morbidity among the three study groups. We used the Chi-square test to test the difference of all rates among the three study groups, used the normal approximate method to calculate the 95% confidence interval of the rates of maternal medical complications, labor and delivery complications, obstetric interventions, and the fetal and neonatal mortality and morbidity. We calculated the relative risk in group 2 and group 3 compared with group 1 for all the outcomes mentioned above. Fetal death was defined as stillbirth weighing 500 grams or more, or if weight was unknown, 20 completed gestation weeks or more. Neonatal death was defined as live-born infant died at 0–27 days of age, and post-neonatal death was defined as those died at 28–364 days of age. Preterm birth (PTB) was defined as gestational age < 37 weeks. Fetal growth restriction (FGR) was defined as less than 10th percentile of birth weight-for-gestational-age z score. The birth weight-for-gestational-age z score was calculated using the following formula: Z = (observed birth weight - mean of birth weight)/SD Where mean and SD was based on all infants in the database, stratified by gender and gestational week. Low birth weight (LBW) was defined as birth weight <2500 grams. Low Apgar score was defined as five minute Apgar score < 7. Anemia was defined as hemoglobin <13. Other fetal and infant outcomes examined included birth injury, hyaline membrance disease, meconium aspiration syndrome, assisted ventilation (<30 minutes and > = 30 minutes), seizures, central nervous system anomalies, circulatory / respiratory anomalies, digestive system anomalies, urogenital system anomalies, musculoskeletal / integumental anomalies, and chromosomal anomalies.
Methods
We used the matched multiple birth file created by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention [ 15 ]. The multiple birth file in 1995–1997 in the United States were linked infant deaths information according to uniform coding specifications, which have undergone vigorous editing and reviewing during the process of the record linkage. Sets of multiples in the 1995–1997 birth file were matched by plurality, state and county of occurrence of delivery, mother's date of birth, date of last menstrual period (LMP), number of prenatal visits, level of education, weight gain during pregnancy, and date of delivery. The matching was successful for 98% of the multiple sets [ 15 ]. Available study variables in the database include socio-demographic information of the parents, maternal life-style factors such as smoking during pregnancy, obstetric history, complications of the pregnancy, labor and delivery, birth weight, gestational age, and other infant outcome variables. Only complete twin sets were included in the current study, with twin set as the unit of analysis in comparison of maternal characteristics and outcomes, and randomly selected one twin in each twin sets as the unit of analysis for fetal and neonatal outcomes.There were two variables for the partners in the data: partner's age and race. The eligible study subjects were divided into three groups according to the availability of the two variables of the partners: both available (group 1), one missing (group 2), and both missing (group 3).We first described the distribution of the maternal characteristics (age, race, marital status, education, cigarette smoking, place of birth, and prenatal care service) of the three study groups. The prenatal care service was defined as adequate, intermediate, and inadequate according to the time of prenatal care visit initiation and the number of prenatal visit using the method described by Kessner et al [ 16 ]. If the first prenatal care initiated at third trimester, or the times of prenatal visit less than 5 in 34 gestation weeks, 4 in 32–33 weeks, 3 in 30–31 weeks, or 2 in 22–29 weeks, the prenatal care service was defined as inadequate; if the first prenatal care initiated at first trimester, and the times of prenatal visit more than 8 in 36 gestation weeks, 7 in 34–35 weeks, 6 in 32–33 weeks, 5 in 30–31 weeks, 4 in 26–29 weeks, 3 in 22–25 weeks, or 2 in 18–21 weeks, the prenatal care service was defined as adequate; and the remainders were defined as intermediate. We compared the rates of maternal medical complications (anemia, cardiac disease, acute or chronic lung disease, diabetes, genital herpes, hydramnios, oligohydramnios, hemoglobinopathy, chronic hypertension, pregnancy-associated hypertension, eclampsia, incompetent cervix, renal disease, RH sensitization, and uterine bleeding), labor and delivery complications (febrile (any temperature reading >38°C), meconium, premature rupture of membrane (>12 hours), abruption placenta, plancenta previa, seizures during labor, precipitous labor (2 nd  stage <3 hours), prolonged labor (2 nd  stage >20 hours), dysfunctional labor, breech, malpresentation, cephalopelvic disproportion, cord prolapse, anesthetic complications, and fetal distress), and obstetric interventions (cesarean section, vacuum / forceps, and labor inductions) among the three study groups. Finally we compared the rates of fetal and neonatal mortality and morbidity among the three study groups. We used the Chi-square test to test the difference of all rates among the three study groups, used the normal approximate method to calculate the 95% confidence interval of the rates of maternal medical complications, labor and delivery complications, obstetric interventions, and the fetal and neonatal mortality and morbidity. We calculated the relative risk in group 2 and group 3 compared with group 1 for all the outcomes mentioned above. Fetal death was defined as stillbirth weighing 500 grams or more, or if weight was unknown, 20 completed gestation weeks or more. Neonatal death was defined as live-born infant died at 0–27 days of age, and post-neonatal death was defined as those died at 28–364 days of age. Preterm birth (PTB) was defined as gestational age < 37 weeks. Fetal growth restriction (FGR) was defined as less than 10th percentile of birth weight-for-gestational-age z score. The birth weight-for-gestational-age z score was calculated using the following formula:Z = (observed birth weight - mean of birth weight)/SDWhere mean and SD was based on all infants in the database, stratified by gender and gestational week. Low birth weight (LBW) was defined as birth weight <2500 grams. Low Apgar score was defined as five minute Apgar score < 7. Anemia was defined as hemoglobin <13. Other fetal and infant outcomes examined included birth injury, hyaline membrance disease, meconium aspiration syndrome, assisted ventilation (<30 minutes and > = 30 minutes), seizures, central nervous system anomalies, circulatory / respiratory anomalies, digestive system anomalies, urogenital system anomalies, musculoskeletal / integumental anomalies, and chromosomal anomalies.Results There were 304466 twins (152233 twin pairs) in the database. Of which, father's age and race were recorded in 259302 twins (129651 twin pairs, 85.17%, group 1), father's age or race was missing in 6036 twins (3018 twin pairs, 1.98%, group 2), and both father's age and race were missing in 39128 twins (19564 twin pairs, 12.85%, group 3). Table  1  describes the distribution of maternal characteristics and prenatal care service among the three study groups. The proportions of teenage, blacks, unmarried, lower education, cigarette smoking during pregnancy, and inadequate perinatal care service were significantly higher in group 2 and (especially) group 3 as compared with group 1 (Table  1 ). Table 1 The distribution (%) of baseline maternal characteristic among three study groups Partner information available (N = 129651) Partner information partly missing (N = 3018) Partner information totally missing (N = 19564) Age* <20 y 5.1 13.7 21.4 20 y- 75.9 72.2 69.4 35 y- 19.0 14.1 9.2 Race* white 84.0 69.0 46.1 Black 12.1 26.3 51.3 Other 3.9 4.7 2.6 Marital status* Married 82.6 39.0 4.1 Unmarried 17.4 61.0 95.9 Education* <12 y 13.1 28.4 36.1 12 y 30.0 31.2 39.9 13–15 y 24.1 15.2 15.9 16 y 19.6 5.8 2.9 >16 y 13.2 19.4 5.2 Cigarette smoking* Yes 8.1 14.9 19.1 No 70.5 55.8 69.5 Not available# 21.3 29.4 11.4 Prenatal care service* Adequate 79.4 55.5 54.1 Mediate 17.8 37.0 34.6 Inadequate 2.7 7.5 11.3 Place of birth* Native 84.5 72.7 88.1 Foreign 15.5 27.3 11.9 *Compared group 2 and group 3 with group 1 with Chi-square test, p < 0.001 # The state of California did not send data on smoking, and all births to residents in California were classified as not available. Table  2  compares the rate of maternal morbidity, labor and delivery complications, and obstetric interventions among the three study groups. The rates of anemia, acute or chronic lung disease, hemoglobinopathy, chronic hypertension, eclampsia, renal disease, meconium, premature rupture of membrane, abruption placenta, and precipitous labor were significantly higher, and the rates of cardiac disease, diabetes, RH sensitization, placenta previa, dysfunctional labor, cephalopelvic disproportion, cesarean section, vacuum / forceps delivery, and induction were significantly lower in group 2 and (especially) group 3 than in group 1 (Table  2 ). Table 2 Comparison of maternal medical complication, labor complication, and obstetric intervention among three study groups (%, 95%CI) Partner information available (N = 129651) Partner information partly missing (N = 3018) Partner information totally missing (N = 19564) Medical risk factors Anemia 2.81 (2.72 – 2.90) 4.27 (3.55 – 5.00)* 4.67 (4.37 – 4.96)* Cardiac disease 0.64 (0.60 – 0.69) 0.40 (0.17 – 0.62) 0.44 (0.35 – 0.53)* Acute or chronic lung disease 0.90 (0.85 – 0.95) 1.36 (0.95 – 1.77)* 1.64 (1.46 – 1.82)* Diabetes 3.46 (3.36 – 3.56) 3.02 (2.40 – 3.63) 2.15 (1.94 – 2.35)* Genital herpes 0.76 (0.71 – 0.80) 1.06 (0.69 – 1.43) 0.82 (0.70 – 0.95) Hydramnios / Oligohydramnios 1.87 (1.80 – 1.94) 2.95 (2.35 – 3.55)* 2.08 (1.88 – 2.28) Hemoglobinopathy 0.07 (0.06 – 0.08) 0.17 (0.02 – 0.31) 0.19 (0.13 – 0.26)* Chronic hypertension 0.90 (0.85 – 0.95) 0.89 (0.56 – 1.23) 1.20 (1.04 – 1.35)* Pregnancy-associated hypertension 7.61 (7.46 – 7.75) 6.39 (5.52 – 7.27)* 7.38 (7.01 – 7.75) Eclampsia 0.94 (0.89 – 0.99) 1.39 (0.97 – 1.81) 1.22 (1.07 – 1.38)* Incompetent cervix 0.81 (0.76 – 0.85) 0.80 (0.48 – 1.11) 0.72 (0.60 – 0.84) Renal disease 0.28 (0.25 – 0.31) 0.56 (0.30 – 0.83) 0.40 (0.31 – 0.49)* RH sensitization 069 (0.64 – 0.73) 0.53 (0.27 – 0.79) 0.49 (0.39 – 0.58)* Uterine bleeding 1.10 (1.04 – 1.16) 0.83 (0.50 – 1.15) 0.97 (0.83 – 1.10) Complication of labor Febrile 1.36 (1.30 – 1.43) 1.36 (0.95 – 1.77) 1.48 (1.31 – 1.65) Meconium 1.37 (1.31 – 1.43) 2.09 (1.58 – 2.69)* 1.92 (1.73 – 2.11)* Premature rupture of membrane (>12 h) 6.34 (6.21 – 6.47) 7.19 (6.27 – 8.11) 7.91 (7.53 – 8.29)* Abruptio placenta 1.16 (1.10 – 1.22) 1.03 (0.67 – 1.39) 1.44 (1.27 – 1.61)* Placenta previa 0.47 (0.43 – 0.51) 0.50 (0.25 – 0.75) 0.30 (0.22 – 0.38)* Seizures during labor 0.05 (0.04 – 0.06) 0.07 (0.03 – 0.16) 0.08 (0.04 – 0.12) Precipitous labor (<3 hours) 1.42 (1.36 – 1.49) 1.29 (0.89 – 1.70) 1.86 (1.67 – 2.04)* Prolonged labor (>20 hours) 0.56 (0.52 – 0.60) 0.63 (0.35 – 0.91) 0.52 (0.42 – 0.62) Dysfunctional labor 2.34 (2.26 – 2.42) 1.52 (1.09 – 1.96)* 1.97 (1.77 – 2.16)* Breech / malpresentation 21.3 (21.0 – 21.5) 21.4 (20.0 – 22.9) 21.4 (20.8 – 22.0) Cephalopelvic disproportion 0.90 (0.85 – 0.95) 0.76 (0.45 – 1.07) 0.52 (0.42 – 0.62)* Cord prolapse 0.55 (0.51 – 0.59) 0.56 (0.30 – 0.88) 0.65 (0.54 – 0.77) Anesthetic complications 0.07 (0.06 – 0.09) 0.10 (0.01 – 0.21) 0.09 (0.05 – 0.13) Fetal distress 3.20 (3.10 – 3.29) 3.45 (2.79 – 4.10) 3.44 (3.18 – 3.70) Obstetric intervention Cesarean 52.0 (51.7 – 52.3) 46.1 (44.7 – 48.2)* 47.5 (46.8 – 48.2)* Vacuum / Forceps 6.76 (6.62 – 6.89) 5.40 (4.59 – 6.21)* 4.46 (4.17 – 4.75)* Induction 13.0 (12.8 – 13.2) 11.9 (10.7 – 13.0) 10.2 (9.8 – 10.6)* *P < 0.05 for group 2 or group 3 compared with group 1 Table  3  compares the fetal and neonatal mortality and morbidity among the three study groups. The rates of PTB, FGR, LBW, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality were significantly higher in group 2 and group 3 than in group 1; the RRs of PTB, FGR, LBW, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality ranged from 1.08 to 3.86 in group 2 and group 3 compared with group 1 (Table  3 ). The rates of hyaline membrane disease and assisted ventilation (< or > = 30 minutes) were also significantly higher in group 3 than in group 1 (Table  3 ). However there were no statistically significant differences in the rates of congenital anomalies among the three study groups (Table  3 ). Table 3 Comparison of fetal and neonatal mortality and morbidity among three study groups in twins in United States Partner information available (N = 129651) Partner information partly missing (N = 3018) Partner information totally missing (N = 19564) % (95%CI) % (95%CI) RR# % (95%CI) RR# General condition PTB (<37W) 53.5 (53.2 – 53.8) 57.8 (56.0 – 59.7)* 1.08 59.6 (58.9 – 60.3)* 1.11 FGR (<10 percentiles z score) 8.26 (8.10 – 8.41) 10.9 (9.72 – 12.0)* 1.32 12.9 (12.4 – 13.4)* 1.56 LBW (<2500 g) 51.4 (51.1 – 51.7) 60.0 (58.2 – 61.7)* 1.17 64.6 (63.9 – 65.3)* 1.26 Apgar score (<7) 3.23 (3.13 – 3.32) 5.14 (4.35 – 5.92)* 1.59 6.45 (6.11 – 6.79)* 2.00 Fetal mortality 0.85 (0.81 – 0.89) 3.28 (2.65 – 3.90)* 3.86 1.59 (1.43 – 1.75)* 1.87 Neonatal mortality 1.92 (1.85 – 2.00) 3.83 (3.12 – 4.53)* 1.99 3.90 (3.62 – 4.17)* 2.03 Post-neonatal mortality 0.46 (0.43 – 0.50) 0.95 (0.59 – 1.32)* 2.07 1.12 (0.97 – 1.27)* 2.43 Abnormal conditions of the newborn Anemia (HGB.<13) 0.40 (0.36 – 0.43) 0.35 (0.13 – 0.57) 0.88 0.46 (0.36 – 0.55) 1.15 Birth injury 0.16 (0.14 – 0.18) 0.18 (0.02 – 0.33) 1.13 0.17 (0.11 – 0.23) 1.06 Hyaline membrane disease 3.28 (3.18 – 3.38) 2.88 (2.27 – 3.49) 0.88 3.87 (3.60 – 4.14)* 1.18 Meconium aspiration syndrome 0.09 (0.07 – 0.11) 0.11 (0.01 – 0.22) 1.22 0.09 (0.05 – 0.13) 1.00 Assisted ventilation (<30 minutes) 3.45 (3.35 – 3.55) 2.67 (2.08 – 3.26)* 0.77 3.87 (3.60 – 4.14)* 1.12 Assisted ventilation (> = 30 minutes) 3.82 (3.71 – 3.92) 3.23 (2.58 – 3.88) 0.85 5.04 (4.73 – 5.35)* 1.32 Seizures 0.07 (0.05 – 0.08) 0.18 (0.02 – 0.33) 2.57 0.10 (0.06 – 0.15) 1.43 Congenital anomalies Central nervous system anomalies 0.15 (0.13 – 0.18) 0.20 (0.04 – 0.36) 1.33 0.14 (0.09 – 0.19) 0.93 Circulatory / respiratory anomalies 0.43 (0.39 – 0.46) 0.60 (0.32 – 0.87) 1.40 0.45 (0.36 – 0.54) 1.05 Digestive system anomalies 0.11 (0.09 – 0.13) 0.17 (0.02 – 0.31) 1.55 0.11 (0.07 – 0.16) 1.00 Urogenital system anomalies 0.21 (0.19 – 0.24) 0.23 (0.06 – 0.40) 1.10 0.17 (0.11 – 0.23) 0.81 Musculoskeletal / integumental anomalies 0.38 (0.34 – 0.41) 0.43 (0.20 – 0.66) 1.13 0.40 (0.31 – 0.49) 1.05 Chromosomal anomalies 0.10 (0.09 – 0.12) 0.17 (0.02 – 0.31) 1.70 0.08 (0.04 – 0.12) 0.80 ** P < 0.05 for group 2 or group 3 compared with group 1; #: relative risk compared with group 1.
Results
There were 304466 twins (152233 twin pairs) in the database. Of which, father's age and race were recorded in 259302 twins (129651 twin pairs, 85.17%, group 1), father's age or race was missing in 6036 twins (3018 twin pairs, 1.98%, group 2), and both father's age and race were missing in 39128 twins (19564 twin pairs, 12.85%, group 3).Table  1  describes the distribution of maternal characteristics and prenatal care service among the three study groups. The proportions of teenage, blacks, unmarried, lower education, cigarette smoking during pregnancy, and inadequate perinatal care service were significantly higher in group 2 and (especially) group 3 as compared with group 1 (Table  1 ).The distribution (%) of baseline maternal characteristic among three study groups*Compared group 2 and group 3 with group 1 with Chi-square test, p < 0.001# The state of California did not send data on smoking, and all births to residents in California were classified as not available.Table  2  compares the rate of maternal morbidity, labor and delivery complications, and obstetric interventions among the three study groups. The rates of anemia, acute or chronic lung disease, hemoglobinopathy, chronic hypertension, eclampsia, renal disease, meconium, premature rupture of membrane, abruption placenta, and precipitous labor were significantly higher, and the rates of cardiac disease, diabetes, RH sensitization, placenta previa, dysfunctional labor, cephalopelvic disproportion, cesarean section, vacuum / forceps delivery, and induction were significantly lower in group 2 and (especially) group 3 than in group 1 (Table  2 ).Comparison of maternal medical complication, labor complication, and obstetric intervention among three study groups (%, 95%CI)*P < 0.05 for group 2 or group 3 compared with group 1Table  3  compares the fetal and neonatal mortality and morbidity among the three study groups. The rates of PTB, FGR, LBW, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality were significantly higher in group 2 and group 3 than in group 1; the RRs of PTB, FGR, LBW, Apgar score <7, fetal mortality, neonatal mortality, and post-neonatal mortality ranged from 1.08 to 3.86 in group 2 and group 3 compared with group 1 (Table  3 ). The rates of hyaline membrane disease and assisted ventilation (< or > = 30 minutes) were also significantly higher in group 3 than in group 1 (Table  3 ). However there were no statistically significant differences in the rates of congenital anomalies among the three study groups (Table  3 ).Comparison of fetal and neonatal mortality and morbidity among three study groups in twins in United States** P < 0.05 for group 2 or group 3 compared with group 1; #: relative risk compared with group 1.Discussion Our large population based study found that the risks of fetal and infant mortality, low birth weight, preterm birth, fetal growth restriction, Apgar score <7, and the need for mechanical ventilation were increased in infants born to mothers with missing information on partner's age or race, especially when both variables of the partner were missing. The women reported no information (or reported only part of the information) on partners appear to have higher prevalence / incidence of most of the maternal morbidity and obstetric complications than in mothers with available information on partners. The poorer pregnancy outcomes observed in women with partner's information been partly and (especially) totally missing than in women with available information on partners were due largely to known important socioeconomic factors for adverse pregnancy outcomes. Many of these women were teenagers, black race, unmarried, with low education, high frequency of cigarette smoking, and inadequate prenatal care services. Teenagers [ 17 ], black race [ 18 ], unmarried [ 19 ], low education [ 3 ], cigarette smoking [ 4 ], and inadequate prenatal care services [ 20 ] are known risk factors of adverse pregnancy outcomes. Our findings that the rates of FGR, LBW, Apgar score <7, hyaline membrane disease, and assisted ventilation were significantly higher in the group with paternal information totally missing than the group of only partly missing suggest that the totally missing group is a particularly high risk group, whereas in the partly missing group, the missing may have occurred in randomly. Exceptions did occur, however. For example, the occurrences of several maternal medical conditions, such as cardiac disorders, diabetes, and RH sensitization, were actually lower in women with no available information on partners (group 2 and 3) than women with available information on partners (group 1). This may, in part, be due to the younger maternal age in groups 2 and 3 than in group 1. The risk of cardiac disorders and diabetes increases rapidly with advancing age [ 21 , 22 ]. Younger mothers are less likely to have had previous pregnancy and therefore less chance for RH sensitization [ 23 ]. The rates of obstetric interventions, including cesarean section, vacuum / forceps, and induction delivery, were lower in groups 2 and 3 than in group 1, which could be explained by the younger age of this group as cesarean section is less common in young women [ 24 ]. The rates of placenta previa and cephalopelvic disproportion were lower in women in groups 2 and 3 than in group 1, this could be explained by the younger age and lower parity of these groups as placenta previa and cephalopelvic disproportion are less common in younger and lower parity women [ 25 , 26 ]. We do not know why the rates of various congenital anomalies were not higher in infants born to mothers with no available information on fathers, despite their much higher risks of fetal and infant mortality and neonatal morbidity. Our study was based on birth certificates, and the diagnosis of many birth defects may be difficult at birth, especially when there is a lack of access to quality perinatal care services such as high resolution ultrasound [ 27 ], although differences in maternal age may play a role here. Our study examined the difference of adverse pregnancy outcomes among three study groups in twins only. It is biologically plausible to apply the study findings to singletons and higher order of multiple pregnancies as well, although the magnitude of the differences may be smaller in singletons and larger in higher order of multiple pregnancies than in twins. Since in the database, there was no indication why a particular variable was not recorded, we can only speculate the reason of missing partner's information. Corresponding variables (i.e., age and race) for the mothers, including those mothers with missing information on partners, were completely recorded. These women may have no relationship or only remote relationship with their partners, as suggested by the extraordinarily high unmarried rates (61% for women missing one variable on their partners and 96% for women missing both variables on their partners). As a result, they may have difficulty or for unknown reasons they may be reluctant to provides partner's information. The paternal information missing may be a comprehensive measure and may be related with age, education, socioeconomic status, race, religion background, or even the reporting quality in different place. Our study only described the association between the paternal information missing and the adverse pregnancy outcomes. Exploration of the reasons of missing partner information and its relationship with adverse pregnancy outcomes require in-depth analysis of data with relevant information.
Discussion
Our large population based study found that the risks of fetal and infant mortality, low birth weight, preterm birth, fetal growth restriction, Apgar score <7, and the need for mechanical ventilation were increased in infants born to mothers with missing information on partner's age or race, especially when both variables of the partner were missing. The women reported no information (or reported only part of the information) on partners appear to have higher prevalence / incidence of most of the maternal morbidity and obstetric complications than in mothers with available information on partners. The poorer pregnancy outcomes observed in women with partner's information been partly and (especially) totally missing than in women with available information on partners were due largely to known important socioeconomic factors for adverse pregnancy outcomes. Many of these women were teenagers, black race, unmarried, with low education, high frequency of cigarette smoking, and inadequate prenatal care services. Teenagers [ 17 ], black race [ 18 ], unmarried [ 19 ], low education [ 3 ], cigarette smoking [ 4 ], and inadequate prenatal care services [ 20 ] are known risk factors of adverse pregnancy outcomes.Our findings that the rates of FGR, LBW, Apgar score <7, hyaline membrane disease, and assisted ventilation were significantly higher in the group with paternal information totally missing than the group of only partly missing suggest that the totally missing group is a particularly high risk group, whereas in the partly missing group, the missing may have occurred in randomly.Exceptions did occur, however. For example, the occurrences of several maternal medical conditions, such as cardiac disorders, diabetes, and RH sensitization, were actually lower in women with no available information on partners (group 2 and 3) than women with available information on partners (group 1). This may, in part, be due to the younger maternal age in groups 2 and 3 than in group 1. The risk of cardiac disorders and diabetes increases rapidly with advancing age [ 21 , 22 ]. Younger mothers are less likely to have had previous pregnancy and therefore less chance for RH sensitization [ 23 ]. The rates of obstetric interventions, including cesarean section, vacuum / forceps, and induction delivery, were lower in groups 2 and 3 than in group 1, which could be explained by the younger age of this group as cesarean section is less common in young women [ 24 ]. The rates of placenta previa and cephalopelvic disproportion were lower in women in groups 2 and 3 than in group 1, this could be explained by the younger age and lower parity of these groups as placenta previa and cephalopelvic disproportion are less common in younger and lower parity women [ 25 , 26 ]. We do not know why the rates of various congenital anomalies were not higher in infants born to mothers with no available information on fathers, despite their much higher risks of fetal and infant mortality and neonatal morbidity. Our study was based on birth certificates, and the diagnosis of many birth defects may be difficult at birth, especially when there is a lack of access to quality perinatal care services such as high resolution ultrasound [ 27 ], although differences in maternal age may play a role here.Our study examined the difference of adverse pregnancy outcomes among three study groups in twins only. It is biologically plausible to apply the study findings to singletons and higher order of multiple pregnancies as well, although the magnitude of the differences may be smaller in singletons and larger in higher order of multiple pregnancies than in twins.Since in the database, there was no indication why a particular variable was not recorded, we can only speculate the reason of missing partner's information. Corresponding variables (i.e., age and race) for the mothers, including those mothers with missing information on partners, were completely recorded. These women may have no relationship or only remote relationship with their partners, as suggested by the extraordinarily high unmarried rates (61% for women missing one variable on their partners and 96% for women missing both variables on their partners). As a result, they may have difficulty or for unknown reasons they may be reluctant to provides partner's information. The paternal information missing may be a comprehensive measure and may be related with age, education, socioeconomic status, race, religion background, or even the reporting quality in different place. Our study only described the association between the paternal information missing and the adverse pregnancy outcomes. Exploration of the reasons of missing partner information and its relationship with adverse pregnancy outcomes require in-depth analysis of data with relevant information.Conclusions Women with no available information (or partly) on partners appear to have higher risks of developing adverse pregnancy outcomes. For this reason, extra attention for these women and their offspring by health care providers and public health workers is needed.
Conclusions
Women with no available information (or partly) on partners appear to have higher risks of developing adverse pregnancy outcomes. For this reason, extra attention for these women and their offspring by health care providers and public health workers is needed.Competing interests The authors declare that they have no competing interests.
Competing interests
The authors declare that they have no competing interests.Authors' contributions Hongzhuan Tan carried out the data analysis and drafted the manuscript. Shi Wu Wen designed the study and helped in results interpretation. Mark Walker participated in the sequence alignment and participated in the results interpretation. Kitaw Demissie participated in the study design and results interpretation.
Authors' contributions
Hongzhuan Tan carried out the data analysis and drafted the manuscript. Shi Wu Wen designed the study and helped in results interpretation. Mark Walker participated in the sequence alignment and participated in the results interpretation. Kitaw Demissie participated in the study design and results interpretation.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here: