Implantation is a process by which the embryo is embedded and fixed with the endometrium of the uterus. At the time of implantation, the embryo is in the form of a blastocyst. Blastocyst surrounded by zona pellucida enters uterus on the sixth day. Zona pellucida prevents it from sticking to the wall of the uterus. As the blastocyst enlarges, zona pellucida covering it becomes stretched and ultimately disappears. Consequently, trophoblast is exposed.
Cells of trophoblast divide mitotically and form new cells that lose their cell membranes and form a mass of cells called syncytiotrophoblast. It invades the endometrium, blastocyst goes deeper and deeper until it completely lies within the endometrium.