Within 11 days of fertilization, the trophoblast forms two layers, 1st the cytotrophoblast and 2nd the syncytiotrophoblast, containing lacunae. The placenta forms a barrier to permit exchange of nutrients, gases and wastes with only slight mixing of fetal blood with maternal blood. Fetal blood cells can be found normally in the maternal circulation in all cases.

As the lacunae enlarge, the trophoblast forms villi, which consist of a vascularized core of cytotrophoblast covered by syncytiotrophoblast. The trophoblast erodes the maternal spiral arteries which then flow directly into the interval of spaces. The fully developed placenta consists of the following 3 layers of membranes 1st the inner amnion, which is a single layer of ectodermal epithelium completely enclosing the embryo; 2nd the outer chorion which surrounds the amniotic sac and includes the villi and trophoblast and 3rd the decidua of the maternal endometrium.