Abnormal genes or chromosomes Most miscarriages occur because the fetus isn't developing normally. In this situation the embryo is present but has stopped developing and died before any symptoms of pregnancy loss have occurred. A molar pregnancy is a noncancerous (benign) tumor that develops in the uterus. A molar pregnancy occurs when there is an extra set of paternal chromosomes in a fertilized egg. This error at the time of conception transforms what would normally become the placenta into a growing mass of cysts. Gabbe SG, et al. Obstetrics: Normal and Problem Pregnancies. Tulandi T, et al. Spontaneous abortion: Risk factors, etiology, clinical manifestations, and diagnostic evaluation. The impact of male factor on recurrent pregnancy loss.