As the embryo and three extra-embryonic cavities increase in size, the chorion bulges further towards the uterine cavity. Initially the chorion is covered by a part of the uterine mucosa (decidua capsularis). The remaining part of the uterine mucosa, at the side where the placenta develops, is called the decidua basalis. The lining of the rest of the uterine surface is called decidua parietalis. Chorionic villi develop more strongly in the region of the decidua basalis (chorion frondosum) than in the region of the decidua capsularis, where they eventually disappear. This part of the chorion becomes smooth in month 3 (chorion laeve).