Excess concepti compared with number of embryos transferred in in vitro fertilization

To reduce the risk of multiple pregnancy, most in vitro fertilization programs limit the number of embryos transferred; because monozygotic twinning may occur, it is possible that more concepti are produced than the number of embryos transferred. To quantify the prevalence of this phenomenon, 1820 clinical pregnancies generated from 11,601 in vitro fertilization treatment cycles were reviewed. Two patients conceived four fetuses each after having had three embryos transferred, while one conceived three fetuses after having had two embryos transferred. The overall prevalence rate was, therefore, one in 607 cases. The obstetric details of these three pregnancies are reported.