The placenta is the structure that brings oxygen and nutrients to a fetus, and gets rid of carbon dioxide and other wastes. The placenta is rich with blood vessels, and by the third month normally releases large amounts of hormones (particularly progesterone) that are necessary to maintain the pregnancy . Placental failure occurs when the placenta fails to produce any progesterone, and usually results in loss (termination) of the pregnancy. See also placental insufficiency .