The placenta is delimited on the fetal side by the chorionic plate and on the maternal side by the decidual plate (basal plate), which is the compact layer of the decidua basalis. During months 4 and 5, septa form in the basal plate. They grow into the intervillous space as placental septa (decidual septa) and partially subdivide it into cotyledons or circulatory units. Each of these cotyledons contains up to four villous trees. Maternal blood from the spiral arteries pours into the cotyledons and flows back into the maternal circulation via the veins of the basal plate. The exchange of gas and substances is performed through the villous trees (placental barrier), which are supplied by the umbilical vessels.
The postnatal placenta is smooth on the fetal side (chorionic plate). The thin layer of amnion allows the vessels to become visible, which run radially to the approximately centrally positioned umbilical cord. The maternal side is equipped with about 10 to 40 slightly elevated areas (cotyledons).