PROGESTINS - The progestins are group of female sex hormones, which produce biological effects similar to those of progesterone. Some examples include hydroxyprogesterone, medroxyprogesterone, megestrol, norethindrone, norgestrel, and progesterone. Progesterone is a natural hormone produced by the corpus luteum of the ovary that has many effects in the body. It allows the maturing of the uterine lining in pregnancy, stabilizes blood vessels, enhances bone formation, stabilizes breast tissue, and many other actions. The addition of progesterone to the treatment decreases the risk of endometrial (lining of the uterus) cancer that is seen with estrogen replacement therapy alone.