What Could Be The Reason For Fetus's Heart To Stop Beating?

Hello. I had abortion 4 days before. My last period was July 9th. In caculation I was in 11 week of pregnancy, but utrasound showed 9 week and 6 days. I went to regular control and doctors have seen stop heart beating of my baby. I didnt have any problems, pains or etc. That night they done abortion. My health was ideal during pregnancy and after abortion. Please could you tel me reasons of stop beating baby's heart?