By the end of the third week, embryonic blood is circulating through the capillaries of the villi. As the growth continues, the villi on the decidua capsularis (abembryonic) pole degenerate to form the chorion laeve, while the villi adjacent to the decidual plate rapidly grow and expand to form the chorion frondosum. The chorion frondosum forms up the placenta together with the decidual plate.
By the end of the fourth month of the pregnancy, the placenta has attained its definitive form and undergoes no further anatomical modifications. It has two components: the maternal portion, formed by the decidual plate, and a fetal portion, made by the chorion frondosum. Growth continues by further ramification of the stem villi into the surrounding intervillous spaces. On the maternal side decidual septa extend into the intervillous spaces, dividing placenta into 10-38 cotyledons.