Estrogen and progesterone are the two main female hormones produced by the ovary. Compounds similar to progesterone, but not identical, are called progestins. Estrogen has, among many other effects, a stimulation effect on growth of the lining of the uterus. If this growth gets out of hand, pre-cancer or cancer of the uterus can occur. This effect makes the use of estrogen alone a risky choice in a woman who has not had a hysterectomy. Progestins reverse the growth effect and help prevent cancer of the uterus. In fact, the risk of cancer is less in women using both estrogen and progestin than in women using no hormones. Any irregular or unusual bleeding is an important symptom to report to your clinician. The large number of estrogen and progestin medications on the market can cause confusion about which is best for a particular person. Some estrogen brands contain many different derivatives of estrogen (conjugated estrogens or esterified estrogens). The most biologically active estrogen is 17-beta-estradiol, usually referred to as estradiol. Estradiol is available in a pure form, identical to that produced by the human ovary. Although estradiol is synthesized, it is in many ways the most natural of all available estrogens. All available estrogen medications are either processed or partially synthesized from animal or plant sources. The choice of estrogen is based on the experience of the clinician and the preference of the person being treated, and may be changed depending on the person's response. (See our web page "Frequently Asked Questions") Progestins are the other major female hormone. Progesterone is the natural progestin produced by the ovary. In the past, this hormone was not available in an orally absorbed form. For this reason, most progestin prescriptions were for medroxy-progesterone acetate (MPA).The brand name for MPA was originally Provera. MPA is still widely used. Progesterone, synthesized from plant sources, is now available in a micronized form. Micronized progesterone is absorbed orally. A well-known study (PEPI trial) showed less reduction in good cholesterol (HDL) with progesterone than with MPA. There is not sufficient evidence, however, to recommend that everyone change from MPA to progesterone. Another progestin is norethindrone acetate (NETA). NETA is available in an oral form and as a combination with estradiol in a skin patch. More than one factor is usually considered in the choice of the progestin recommended. Women who have had a hysterectomy usually do not need progestin along with estrogen. There are instances, however, when even a woman who has undergone a hysterectomy will benefit from progestin. These situations include cases when the woman has a history of uterine cancer, certain ovarian tumors, or high triglyceride levels. Some experts recommend using a progestin in women who have had endometriosis, even if they have had a hysterectomy.