How long after conception does the fertilized egg implant?

The fertilized conceptus enters the uterus as a 2- to 8-cell embryo and freely floats in the endometrial cavity about 90-150 hours, roughly 4-7 days after conception. Most embryos implant by the morula stage, when the embryo consists of many cells. This happens, on average, 6 days after conception. The new embryo then induces the lining changes of the endometrium, which is called decidualization. It then rapidly begins to develop the physiologic changes that establish maternal-placental exchange. Prior to this time, medications ingested by the mother typically do not affect a pregnancy.