Human placenta, illustration. This is the fetal side of the placenta. The cut membranes are the amnion, chorion and the decidua (of the uterus). The amnion covering the umbilicus has been removed, showing the umbilical vein (blue) and the two umbilical arteries (red). Large chorionic vessels on the placenta surface converge to the umbilical cord. Compartments of the placental tissue are called cotyledons. At birth the placenta is 15-25 cm in diameter, 3 cm thick, and weighs about 500 g. From Lizars, J. 1823 A system of anatomical plates of the human body. W. H.Lizars, Edinburgh.