Illustration of the human placenta. The placenta is an organ that develops in the female uterus during pregnancy. Through it, and an umbilical cord (at lower right), the blood supply of the mother is linked to the fetus. A rich supply of placental blood vessels (red and blue, at centre) allow for nutrients and oxygen to be transferred from mother to fetus. And it allows waste products from the fetus to pass back to the mother. Apart from being a vital link between mother and fetus, the placenta also produces hormones. By the end of pregnancy it has grown about 20cm wide and 2.5cm thick. Shortly after birth, the placenta is expelled from the uterus as the afterbirth.