During the first 14 days of its development, the product of the fertilized egg is variously termed the conceptus, the zygote, the blastocyst, or the "preembryo." It is after implantation that what is properly termed the embryo forms in the center of the conceptus; the remaining 99 percent of the tissue gives rise to the placenta and other extraembryonic membranes. The embryo develops into a fetus at about 8 weeks after fertilization or 10 weeks after the last menstrual period (Cook 1989).

The first laboratory and clinical data on RU 486 were presented by Herrman et al. in 1982.