Progesterone is a hormone produced mainly by the ovaries and the placenta in female animals during the period when they are able to bear young. This hormone can also be produced by the adrenal glands in both the females and males by the testes in males. It plays an important role in the normal menstrual cycle and in pregnancy. When the egg leaves its follicle in the ovary, the follicle is transformed into the corpus luteum. The corpus luteum manufactures progesterone, apparently from cholesterol. Progesterone then acts upon the uterine lining, which has already been primed by the female sex hormone, estrogen, in preparation for the egg. If the egg is not fertilized, progesterone production ceases, menstruation occurs, and the corpus luteum degenerates; if the egg is fertilized, the corpus luteum continues to secrete progesterone. This maintains pregnancy, prevents egg from maturing in ovary, inhibits menstruation, and stimulates growth of the breasts. Later in pregnancy progesterone manufacture is taken over by placenta, the structure through which the developing fetus receives nutrition.
Physicians use progesterone as a drug to treat disorders of the reproductive system, such as premenstrual tension, infertility, and miscarriage. Synthetic forms of progesterone are used alone and in combination with synthetic estrogen in birth control pills.
