Progesterone is a hormone released by the corpus luteum in the ovary and plays an important role in regulating the menstrual cycle and in maintaining early stages of pregnancy. It may also be involved in the growth of certain cancers.

During a woman’s menstrual cycle, at ovulation, an egg is released from the ovary and the remnants of the ovarian follicle that enclosed the developing egg forms a structure called the corpus luteum. The corpus luteum releases progesterone and to a lesser extent, estradiol to prepare the body for pregnancy in the event that the released egg is fertilized. Progesterone’s function is to maintain the uterine lining throughout the pregnancy and the levels increase from 9 to 32 weeks. If no fertilized egg implants, progesterone levels drop and menstruation begins.

There is no medical consequence of producing too much progesterone, however high levels of progesterone are associated with various conditions including congenital adrenal hyperplasia and an increased risk for breast cancer. If progesterone is absent or its levels are too low, irregular and heavy menstrual bleeding can occur and an ovary may fail to release an egg at ovulation, which occurs in women with polycystic ovary syndrome. A drop in progesterone during pregnancy can also result in a miscarriage and early labor.