Chorion

For the chorion in vertebrate eggs, see Chorion (egg).

For the entertainment company see Chorion (company).

In humans and most mammals, the chorion is one of the membranes that exist during pregnancy between the developing fetus and mother (the fetal membranes). The chorion and the amnion together form the amniotic sac.

It is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.