Progesterone is a steroid hormone synthesized from cholesterol and is important as an intermediate in the pathway to cortisol via pregnenolone, estrogens and testosterone.

Progesterone induces the cyclic changes in the endometrium that allow implantation of the fertilized ovum. Progesterone is also responsible for maintenance of the uterus during pregnancy, suppression of uterine contractions until just prior to parturition and preparation of the breasts for lactation. Pregnanediol is an inactive product of progesterone metabolism. Urinary pregnanediol is an indirect measure of progesterone levels in the body. In women, progesterone is produced mainly by the corpus luteum following ovulation. Some progesterone is also produced by the adrenal cortex.

Progesterone is naturally secreted by the ovary in the second two weeks of the menstrual cycle in reproductive age ovulating women. Progesterone or progesterone-like substances called progestogens or progestins are also ingested by women in birth control pills, menopausal hormone replacement therapy, or just sometimes to induce a menstrual period or regulate abnormal bleeding problems if menses are skipping or bleeding is irregular or prolonged. Progesterone has been used also as therapy for PMS syndrome and for women with infertility or frequent pregnancy loss.

Progesterone has the identical chemical structure to the substance made in a woman's body by the ovarian corpus luteum (gland formed after an egg is ovulated each month). Actually the progesterone is now synthetically made but it behaves as best we know, just like the body's natural progesterone once it is absorbed into the blood stream. This is to be distinguished from synthetic progesterone-like chemicals called progestogens which bind to the body's progesterone receptors and function for the most part, just like progesterone. Because they are chemically different than natural progesterone, they sometimes have side effects or actions that are different than progesterone.

Progesterone is one of the hormones that affects the female reproductive system. Its levels vary during the menstrual cycle and it is an ingredient of most birth control pills. It has a number of physiological effects, usually to normalize or restore changes to the body caused by estrogen. These effects include normalizing blood clotting and vascular tone, zinc and copper levels, cell oxygen levels, and use of fat stores for energy. Progesterone also assists in thyroid function and osteoblast bone building, and appears to prevent endometrial (cancer involving the uterine lining) and breast cancer.

Note that progesterone is a steroid hormone, and consists of four interconnected cyclic hydrocarbons. It is a hormone involved in the pregnancy and embryo development of humans. The chemical itself contains the ketone and aldehyde functional groups, as well as two methyl branches. Like all steroid hormones, it is hydrophobic. This is mostly due to its lack of very polar functional groups.

During the follicular phase of the menstrual cycle, low levels of progesterone are found in serum and saliva. During the luteal phase, the corpus luteum produces progesterone and levels increase sharply for a maximum of 5 to 10 days. If implantation of a fertilized ovum does not occur, a steep decline in these levels is seen at about 4 days prior to the menstrual period. In postmenopausal women it has been shown that a combination of progesterone plus estrogen provides an optimal hormonal profile for prevention of cardiovascular disease, osteoporosis and endometrial protection. Progesterone has also been suggested to alleviate the symptoms of premenstrual syndrome. Measurement of progesterone levels is useful in the detection of luteal insufficiency in the early stages of pregnancy. Measurement of progesterone levels is also useful to monitor women on progesterone replacement therapy for relief of symptoms or prevention of some of the long term problems associated with hormone deficiency, including osteoporosis and heart disease.