Placenta: A temporary organ that joins the mother and fetus, transferring oxygen and nutrients from the mother to the fetus and permitting the release of carbon dioxide and waste products from the fetus. The placenta is roughly disk-shaped, and at full term it measures about 7 inches in diameter and slightly less than 2 inches thick. The upper surface of the placenta is smooth, and the under surface is rough. The placenta is rich in blood vessels. The placenta is expelled with the fetal membranes during the birth process; together, these structures form the afterbirth.