Organ in most mammals that develops in the uterus along with a fetus to mediate metabolic exchange. The umbilical cord attaches it to the fetus at the navel. Nutrients and oxygen in the mother's blood pass across the placenta to the fetus, and metabolic wastes and carbon dioxide from the fetus cross in the other direction; the two blood supplies do not mix. Other substances (e.g., alcohol or drugs) in the mother's blood can also cross the placenta, with effects including congenital disorders and drug addiction in the newborn (see fetal alcohol syndrome); some microorganisms can cross it to infect the fetus, but so do the mother's antibodies. The placenta, weighing a pound or more at the end of pregnancy, is expelled at parturition. Some animals eat it as a source of nutrients; in some species this stimulates lactation.
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