In the latter case, the presence of the IUD could interfere with the fertilized egg’s implanting in the lining of the womb and progressing as a normal pregnancy would.
وَقَالَ ٱخْتِصَاصِيُّونَ كَثِيرُونَ إِنَّهُ يُسَبِّبُ رَدَّ فِعْلٍ فِي ٱلرَّحِمِ يُصَعِّبُ عَلَى ٱلْحَيَوَانِ ٱلْمَنَوِيِّ أَنْ يَصِلَ إِلَى ٱلْبُوَيْضَةِ وَيُلَقِّحَهَا .‏