The unique combination of data from arrested embryos also sheds new light on the still largely mysterious earliest stages of pregnancy through natural means. "It is very surprising that most of these embryo arrests are coming not from errors in egg formation but from errors happening in cell divisions after fertilization. Looking for genetic differences, Johns Hopkins and London Women's Clinic researchers in the UK compared IVF embryos that failed to develop within a few days of fertilization with embryos that survived. "Genetic testing is typically only done on IVF embryos that survive in order to decide which embryo to transfer to the uterus," McCoy said. Scientists have studied aneuploidy for decades by screening IVF embryos, and these mishaps are well understood to be the cause of pregnancy loss in humans.