Progesterone is a hormone that prepares the uterus for pregnancy and maintains pregnancy. It is produced naturally by the ovaries and placenta during pregnancy. Synthetic progestins are also used as contraceptives and for hormone replacement therapy. Progestins work by converting the estrogen-primed endometrium to a secretory state and maintaining it to support pregnancy. They also have other effects throughout the body. Mifepristone is a progesterone antagonist that is used to terminate early pregnancies by blocking the effects of progesterone and causing abortion.