The placenta, which occurs only in placental mammals, is a specialized structure derived from the lining of the uterus (womb) and part of the embryo after implantation. It separates and yet ensures a close and extensive contact between the maternal (uterine) and fetal (umbilical) blood circulation. This allows nutrients and oxygen to pass from the mother to the fetus, and waste products to pass in the reverse direction. The placenta thus enables the embryo and fetus to live as a parasite, dependent on the maternal organs. Gonadotrophins are produced by the placenta which prepares the maternal body for delivery and mammary glands for lactation. The placenta is delivered after the child at birth (the afterbirth) by separation of the blood vessel layers; placental disorders may cause ante- or postpartum hemorrhage or fetal immaturity.
Placental mammals are mammals whose young develop to an advanced stage attached to the placenta – a life-supporting organ inside the mother's uterus. All mammals except the monotremes and most marsupials are placentals.