The risk of spontaneous first trimester abortion is estimated to be between 10 and 20%. Although it is common knowledge that the incidence of abortion decreases as pregnancy progresses, exact data in relation to the duration of pregnancy are scarce. We reviewed 1597 clinical IVF/ICSI pregnancies with known outcome and tabulated the number of miscarriages or fetal demise per intervals of 2 weeks. We furthermore compared the outcome in terms of fetal survival of 1200 singleton pregnancies with that of 397 twin pregnancies. The overall incidence of non-ongoing singleton pregnancies was 21.7%. Fetal death, after positive heart activity had been recorded, occurred in 12.2% of singleton pregnancies. The overall incidence of spontaneous abortion in twin pregnancies was 17.1% (12.1% vanishing twins and 5.0% complete miscarriages). The incidence of miscarriage in the twin pregnancies, expressed per gestational sac, was 11.1%. Once fetal heart activity was present, the risk of abortion (per gestational sac) was 7.3%, which is significantly lower than that in singleton pregnancies. Our data give an estimate of the probability of miscarriage or fetal demise at any given period of the first trimester both for singleton and twin pregnancies. Twin pregnancies after IVF have a better potential for survival than singleton pregnancies.
