Implantation

Implantation occurs about a week after fertilization when the blastocyst, having hatched from zona pellucida, adheres to endometrial lining primed by progesterone, with trophoblast cells invading to establish placental connections. Apposition involves L-selectin on trophoblast binding receptors, then apposition and adhesion via integrins, followed by syncytiotrophoblast erosion of maternal vessels for nutrient exchange. Decidual reaction supports, and failure leads to miscarriage or ectopic pregnancy if outside uterus.