Implantation

Implantation occurs 6–10 days after ovulation or egg retrieval. The blastocyst hatches from its shell (zona pellucida), contacts the endometrium, and begins invasive attachment — first adhering, then embedding, and finally fully embedding into the decidualized lining. Successful implantation requires a receptive endometrium and a chromosomally normal embryo.

The trophectoderm cells that eventually become the placenta invade the decidua and establish blood supply. Implantation triggers hCG production, which supports the corpus luteum and progesterone output until the placenta takes over. Failure of any step in this cascade leads to either no pregnancy or early pregnancy loss.