ABSTRACT
Embryonic implantation in humans is a progressive and a complex process, in which the embryo has to appose, then has to adhere to the maternal endometrial epithelium (EE), and finally has to invade it. The existence of a receptive phase or “window” for embryo implantation is required to make possible maternal-embryo interaction. The window process lasts approximately four days in women, from days 20-24 of the cycle.1 Timing from the luteinizing hormone (LH) peak, which precedes ovulation by about 36 hours, gives a window lasting from approximately day LH+7 to LH+11.2
The blastocyst in the apposition period faces the specific site, generally in the posterior wall of the uterus. In the next phase of implantation, the adhesion phase, the trophoectoderm of the blastocyst contacts directly with the EE. It occurs between day 6 and 7 after ovulation. This concept is not obvious and more researchers are starting to believe that epithelial penetration without adhesion of the blastocyst to the luminal surface is feasible.3 The last phase is the invasion of the embryonic trophoblast, traversing adjacent cells of the epithelial lining, and invasion into the endometrial stroma up to the uterine vessels. The blastocyst-induced apoptosis on EE seems to be a plausible mechanism to allow the embryo to traverse the epithelial barrier before it can invade deep into the stroma.4
