Accessory placenta: An extra placenta separate from the main placenta. Also called a succenturiate or supernumerary placenta. The placenta is the organ joining the mother and fetus, the organ that permits the provision of oxygen and nutrients to the fetus and the release of carbon dioxide and waste products from the fetus to the mother. The word "placenta" means a flat cake. The main placenta is disk-shaped and at full term measures about 7 inches (18 cm) in diameter and a bit less than 2 inches (4 cm) thick. The upper surface of the placenta is smooth while the under surface is rough. The placenta and the fetal membranes are the afterbirth.