postimplantation

Postimplantation refers to the period after a blastocyst has implanted into the endometrium, marking the transition from nidation to early embryonic development. Implantation typically occurs about six to ten days after ovulation, with clinical pregnancy dating beginning around the third week of gestation. During this phase the endometrium undergoes decidualization to support the implanted embryo, and the trophoblast differentiates to form structures that will become the placenta.

Key processes during postimplantation include the formation of the maternal–fetal interface and the establishment of placental precursors. The syncytiotrophoblast begins to secrete human chorionic gonadotropin (hCG), which helps maintain the corpus luteum and progesterone production in early pregnancy. The maternal immune response is modulated to tolerate the semi‑foreign embryo, and the decidua reorganizes to support ongoing growth. Trophoblast invasion and villus formation begin, setting the stage for placental development and nutrient exchange.

As postimplantation progresses, placental circulation starts to form, enabling exchange of nutrients, gases, and waste between mother and embryo. The embryo enters a phase of rapid development, with early organogenesis occurring in the weeks that follow and the placenta gradually assuming a major role in sustaining pregnancy.

Clinical relevance centers on confirming ongoing pregnancy and monitoring early development. Serum hCG levels and early ultrasound findings help verify progression beyond implantation. Complications during this period, such as implantation failure, miscarriage, or ectopic pregnancy, have significant implications for pregnancy outcomes and clinical care.