The placenta is an organ that sustains and nourishes the fetus and is of both fetal and maternal origin. The placenta produce hormones and thus has an endocrine function. The placenta has an immunological role in the suppression of fetal rejection. Blood travels from the fetus to the placenta and returns to the fetus via the umbilical cord which attaches near the center of the placenta. The placenta is expelled from the uterus following birth of the baby and is thus known as the afterbirth. The fully developed placenta is discoidal in shape with a diameter of 6-8 in (15-20 cm) and a thickness of about 1 in (2-3 cm).

The fetus has lungs but they have no respiratory function. The placenta serves in lieu of lungs. Fetal blood moves to the placenta via two umbilical arteries. Fetal blood loses carbon dioxide in the placenta and oxygen derived from...