Afterbirth: The placenta and fetal membranes that are expelled from the uterus following the baby's birth. Hence, the "afterbirth." The placenta is what joins the mother and fetus. It also permits the flow of oxygen and nutrients to the fetus besides the release of carbon dioxide and waste products from the fetus to the mother. As a uniquely disposable organ, the placenta is disk-shaped and at full term, measures about 7 inches (18 cm) in diameter and little under less than 2 inches (4 cm) thick. The fetal membranes - the chorion is the outer one and the amnion is the inner one - envelop the embryo and contain the amniotic fluid. The word "afterbirth" entered the English language in the 16th century. The term has also been applied to a child born after the father's death or last testament.