Synthetic versions of progesterone, called progestins, are included in oral contraceptives and hormone replacement therapy along with estrogen. Progestins are also used alone for birth control and for treatment of a variety of conditions, including abnormal uterine bleeding and amenorrhea (absence of periods); endometriosis; breast, kidney or uterine cancer; and appetite and weight loss related to AIDS. Progestins may also be used as an aid in measuring estrogen's effect in the uterus.