placenta

placenta (plusen'tu) [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. The placenta is attached to the uterus, and the fetus is connected to the placenta by the umbilical cord. The placenta draws 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, and gonadotrophin. 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.