Progesterone is a hormone produced in the body which helps to regulate the menstrual cycle of women. Men also produce a small amount of progesterone, but it is less important to sexual maturity in men than is testosterone. Chemical forms of progesterone are widely used by women as part of birth control. The unique aspects of progesterone tend to explain their effectiveness as a birth control method.

In women, progesterone is produced just before ovulation in order to enhance the possibility of becoming pregnant. Rise of progesterone levels prior to ovulation increases the body temperature slightly, creates more vaginal mucus which makes sperm more likely to survive to reach and fertilize an egg, and makes the uterus muscles less likely to contract. If a woman does become pregnant, main production of progesterone switches over to the developing placenta around the eighth week of pregnancy.

If a woman does not become pregnant, progesterone levels begin to decline after ovulation, enough so that the uterine lining is shed from the body in the form of the menstrual period. Along with estrogen, progesterone maintains the balance of the women’s menstrual cycle, producing monthly periods or menstruation.

With birth control pills that contain progesterone, or chemicals forms of it called progestin, the body is essentially fooled into thinking it is pregnant, because it detects a higher level of progesterone. This means ovulation does not usually occur. Generally, one stops taking progestin and estrogen for seven days each month in order to induce a period and shed the lining of the uterus. Newer forms of the pill now have women skipping fewer days in order to have fewer periods in a year.

Progesterone also serves several other important functions in the body. Progesterone aids in immunity, reduces swelling and inflammation, stimulates and regulates the production of the thyroid gland, and keeps blood-clotting levels at normal values. Progesterone also can be said to be an “anti-aging” hormone. It keeps bones strong, produces collagen, and helps keep nerves functioning at appropriate levels. Some researchers are now testing the effects of progesterone shots on people with multiple sclerosis, to see if progesterone can help keep nerves and skeletal muscles from deteriorating.

As a woman ages, levels of progesterone and estrogen begin to decline. This gradually leads to menopause. It is easy to see how the decline in these levels not only affects the menstrual cycle but also contributes to aging, because less collagen means less skin elasticity, which contributes to skin wrinkling. Low progesterone levels also decrease the body’s ability to create new bone cells, which puts a woman at an increased risk of developing osteoporosis.

In the past, hormone replacement therapy consisted of replacing lost or diminished estrogen due to menopause. Studies have shown that this has increased risk of breast cancer, and is thus undertaken by fewer women. Medical researchers are now re-examining the possible use of replacing progesterone in both men and women, as a possible means for reducing likelihood of contracting some forms of cancer. As well, progesterone might have a positive effect on women who seek an alternative to estrogen therapy when experiencing menopause, though some of the side effects of progestin are similar to those experienced by women undergoing menopause.