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1. What are the functions of natural progesterone?

   Progesterone helps prepare your body for conception and pregnancy and regulates the monthly menstrual cycle. Progesterone levels rise in the second half of the menstrual cycle.

   One of progesterone's most important functions is its role in thickening the lining of the uterus each month. The enriched endometrial lining is prepared to receive and nourish a fertilized egg.

   If a pregnancy occurs, progesterone is produced in the placenta and levels remain elevated throughout the pregnancy. The combination of high estrogen and progesterone levels suppress further ovulation during pregnancy. Progesterone also encourages the growth of milk-producing glands in the breast during pregnancy.

   If fertilization does not occur, estrogen and progesterone levels drop, the lining of the uterus breaks down, and menstruation occurs.

2. What conditions do progestogens treat?

   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.

3. Why can't I just take natural progesterone?

   Natural progesterone (non-micronized) is not effective orally and natural progesterone used in over-the-counter creams is not as effective as progestins or micronized progesterone. In particular, progesterone creams should not be used as a substitute for progestins in combination with estrogen replacement for treating menopausal symptoms. There is no evidence that they protect against endometrial cancer like progestins or micronized progesterone.

   There are other roles for natural progesterone, however. The U.S. Food and Drug Administration has approved Crinone, a vaginal gel formulation of natural progesterone for treatment of amenorrhea and for infertility procedures, and an intra-uterine device that delivers progesterone directly to the uterus is an effective form of birth control and is sometimes used to treat abnormal uterine bleeding.

4. What side effects might I expect when taking progestin?

   Potential side effects often mimic those of pre-menstrual syndrome, including breakthrough bleeding, menstrual cramps, bloating, dizziness, moodiness and fatigue. Some women experience rarer side effects like depression, excessive breast milk or breast sensitivity, difficulty falling asleep, flushing and changes in body or facial hair. Potential adverse effects of long-term use of progestin with estrogen include invasive breast cancer, heart disease-related events, dementia, including Alzheimer's disease, stroke and blood clots. Talk to your health care professional if you experience side effects. Many fade with continued use, or a different product may help.

5. Why do birth control pills and hormone replacement therapy contain progestogens?

   Estrogen promotes buildup of the endometrium (the uterine lining), which can lead to cancer. Incorporating a progestogen prevents estrogen from building up the lining of the uterus and ensures the excess tissue will be sloughed off.

6. Should I take progestin-based contraceptives instead of combination oral contraceptives that contain estrogen?

   Most oral contraceptives contain a combination of estrogen and progestin. These pills are somewhat more effective than progestin-only mini-pills (although the mini-pills are 99.5 percent effective if used correctly). Mini-pills are a good option if you are unable to take an estrogen-containing pill or are bothered by estrogen-related side effects such as headaches. Progesterone-only pills are appropriate for older women, especially smokers who want to use an oral hormonal contraceptive method, and postpartum and/or breastfeeding women. Other progestin-only alternatives include the long-acting contraceptives Depo-Provera (injection) and Norplant (implants). Both are long acting, reversible, highly reliable and more convenient for some women than taking a daily pill. Two types of intrauterine devices (IUD) also provide long-acting, reversible contraception in part by releasing progestin directly into the uterine lining. Discuss the options with your health care provider to decide what's best for you.

7. How does the mini-pill prevent pregnancy?

   The mini-pill can suppress ovulation, though not consistently. It works mainly by thickening the cervical mucus, preventing sperm from reaching the egg; because the mucus may last only a day, forgetting to take the pill can leave you vulnerable to pregnancy. The mini-pill is 99.5 percent effective if always used correctly.

8. Can progestins cause breast cancer?

   Some studies show a link between progestins and breast cancer. In the studies, women taking combination birth control pills had a higher risk of breast cancer than women taking birth control pills containing estrogen alone. Discuss this risk with your health care provider.