Implantation

Following fertilisation and cleavage, the embryo becomes a morula and then a blastocyst consisting of an inner cell mass (embryoblast) and an outer layer (trophoblast). Around day 6, the blastocyst 'hatches' from the zona pellucida. Implantation usually occurs on the posterior or anterior wall of the uterine body. The trophoblast cells in contact with the endometrium differentiate into two layers: an inner, mitotically active cytotrophoblast and an outer, multinucleated syncytiotrophoblast. The syncytiotrophoblast produces proteolytic enzymes that erode maternal tissue, allowing the embryo to bury itself within the uterine wall. By day 9, the embryo is completely embedded, and the site of penetration is sealed by a fibrin coagulum. This invasive process also establishes the early uteroplacental circulation through the formation of lacunae within the syncytiotrophoblast, which later fill with maternal blood. Successful implantation requires a 'receptive' endometrium, often termed the 'implantation window,' regulated by progesterone and various cytokines.

The molecular dialogue between the blastocyst and the endometrium is essential. Adhesion molecules such as integrins and selectins facilitate the initial 'rolling' and attachment of the blastocyst. The syncytiotrophoblast loses its individual cell boundaries to form a syncytium, which is highly invasive and lacks HLA class I or II antigens, protecting the embryo from maternal immune attack. The maternal stromal cells undergo the decidual reaction, enlarging and accumulating glycogen and lipids to provide nutrients. hCG produced by the syncytiotrophoblast enters the maternal bloodstream and prevents the regression of the corpus luteum, which continues to secrete progesterone to maintain the pregnancy until the placenta takes over (luteal-placental shift).

Failures in implantation are a leading cause of early pregnancy loss. Ectopic pregnancy, where implantation occurs outside the uterus, is a life-threatening emergency. Placenta praevia occurs if implantation happens in the lower uterine segment near the internal os. Clinically, 'implantation bleeding' can sometimes be mistaken for a light period, leading to inaccurate dating of the pregnancy. Detection of hCG is the gold standard for pregnancy testing. In IVF, the 'window of receptivity' is crucial for successful embryo transfer. Modern screening for pre-eclampsia also looks at early trophoblastic invasion as shallow invasion is linked to hypertensive disorders of pregnancy.

Pregnancy is confirmed via urinary or serum beta-hCG. Ultrasound (TVS) is used to locate the gestational sac, which should be visible from 4.5–5 weeks gestation. In suspected ectopic pregnancy, serial hCG levels and ultrasound are primary tools.

For implantation failure in ART, hormonal support (progesterone) is used. Ectopic implantation requires medical or surgical management. Cases of abnormal implantation depth (Placenta Accreta) require specialized obstetric care during delivery to manage haemorrhage risk.