Implantation

Implantation can only occur during the window of implantation (≈days 20–24 of a 28-day cycle), when progesterone has made the endometrium receptive. It proceeds through apposition and adhesion of the hatched blastocyst, embryonic pole first (mediated by integrins, selectins and the loss of anti-adhesive mucins), followed by invasion. Trophoblast at the embryonic pole differentiates into an inner, mitotically active cytotrophoblast and an outer, multinucleated, invasive syncytiotrophoblast, which erodes the epithelium and burrows into the stroma until the embryo lies completely within the endometrium (interstitial implantation), the surface defect sealed by a fibrin plug.

During the second week ("the week of twos"), the trophoblast forms two layers, the embryoblast forms a bilaminar disc (epiblast and hypoblast) with two cavities (amniotic cavity and yolk sac), and lacunae in the syncytiotrophoblast (≈day 9) open into eroded maternal capillaries and glands (≈days 11–12), establishing the primitive uteroplacental circulation. The syncytiotrophoblast secretes hCG from the start, which rescues the corpus luteum. The endometrial stroma responds with the decidual reaction: stromal cells swell with glycogen and lipid, regulate trophoblast invasion and provide nutrition and immune tolerance.