Incidence and causes of pregnancy loss after Day 70 of gestation in Thoroughbreds.
BACKGROUND
Pregnancy loss after Day 70 of gestation manifests as abortion, stillbirth or perinatal death. While previous studies have reported the diagnoses of laboratory submissions, none have quantified the incidence and causes of abortions, stillbirths and perinatal mortality at a population level.


OBJECTIVES
To report the incidence and causes of pregnancy loss after Day 70 of gestation in a cohort of Thoroughbreds.


STUDY DESIGN
Retrospective cohort study.


METHODS
Outcomes of Day 70 pregnancies were collected from eight Thoroughbred farms over the 2013-2017 breeding seasons. Stud, veterinary and laboratory records were supplemented with publicly available data. Cause of loss was categorised using custom criteria.


RESULTS
Data were collected on 3,586 pregnancies from 1,802 mares. The incidence risk of a pregnancy failing to produce a live foal at 24 hours post parturition was 7.3% (95% confidence interval (CI) 6.5-8.2, equating to 7.3 cases per 100 Day-70 pregnancies). The incidence of pregnancy loss between Day 70 and 300 of gestation, Day 301-315 and stillbirth/perinatal death was 4.0% (95% CI 3.4-4.7), 0.3% (95% CI 0.2-0.6) and 1.4% (95% CI 1.1-1.9) respectively. Of the pregnancy losses where tissue was available, 61.1% were submitted for post-mortem examination. The incidence risk of loss due to umbilical cord-related pathologies was 1.5% (95% CI 1.1-1.9), 0.4% (95% CI 0.2-0.6) for noninfectious placental disease and 0.3% (95% CI 0.2-0.6) for both infectious placentitis and Equine Herpesvirus infection. No primary diagnosis was made in 11.2% of the cases which underwent full post-mortem examination.


MAIN LIMITATIONS
It was not possible to differentiate between intra-partum stillbirth and early post-partum death.


CONCLUSION
Pregnancy loss after Day 70 of gestation is a significant source of loss in the Thoroughbred with umbilical cord-related pathologies being the most commonly diagnosed cause. Reporting the incidence of pregnancy loss at a population level with clear case definitions will allow for accurate global comparisons.
['Roach|Jessica M|JM|https://orcid.org/0000-0002-0696-9264', 'Foote|Alastair K|AK|', 'Smith|Ken C|KC|https://orcid.org/0000-0002-4861-7048', 'Verheyen|Kristien L|KL|', 'de Mestre|Amanda M|AM|https://orcid.org/0000-0002-9422-2370']
D000034:Abortion, Veterinary D000818:Animals D005260:Female D006734:Horse Diseases D006736:Horses D015994:Incidence D036801:Parturition D010920:Placenta D011247:Pregnancy D012189:Retrospective Studies
abortion stillbirth placentitis horse umbilical cord