Decidua and placenta

The decidua overlying the chorion frondosum (decidua basalis) anchors firmly to the chorion and forms the decidual plate (basal plate). The decidua located over the abembryonic area and enclosing the chorion laeve like a capsule is called the decidua capsularis. It gets thinner with the growth of the amniotic cavity and finally disintegrates, leaving behind the chorion laeve. From month 4, the amniotic cavity begins to fill the uterine cavity. This causes the chorion laeve to come into contact with the decidua parietalis on the opposite uterine wall and fuse together with it. The chorion frondosum (fetal part) and the decidua basalis (maternal part) together form the placenta.