placenta (pləsĕnˈtə) [key] or afterbirth, organ that develops in the uterus during pregnancy. It is a unique characteristic of the higher (or placental) mammals. In humans it is a thick mass, about 7 in. (18 cm) in diameter, liberally supplied with blood vessels. Composed mainly of tissue that develops from the embryo beginning early in pregnancy, the placenta is attached to the uterus, and the fetus is connected to the placenta by the umbilical cord. The placenta acts as an interface between the mother and fetus, drawing nourishment and oxygen, which it supplies to the fetus, from the maternal circulation. In turn, the placenta receives the wastes of fetal metabolism and discharges them into the maternal circulation for disposal. It also acts as an endocrine gland, producing estrogen, progesterone, gonadotrophin, and serotonin, and works to prevent the mother's immune system from rejecting the fetus. Shortly after delivery of the fetus the placenta is forced out by contractions of the uterus. Severe hemorrhage may occur if the placenta does not emerge in its entirety or if the uterus fails to contract properly.