The Ovarian Cycle

Each month in a woman's reproductive years an egg develops in one of the ovaries and is released to potentially be fertilized and reproduce. So how does this happen and how is it regulated?

The stimulation to the ovary comes from the pituitary gland. The pituitary gland is a small peanut sized gland that is attached to the bottom of the brain, and is housed in a thin-walled bony part of the skull called the sella tursica.

Each month the hypothalamus, the part of the lower brain that regulates pituitary function, senses that there is no pregnancy, and stimulates the pituitary to release a hormone called follicle stimulating hormone, or FSH. The FSH stimulates several germ cells in the ovaries to start to develop. Over a few day's time one of these developing eggs becomes dominant and the others stop developing and atrophy. The dominant follicle becomes larger and about 14-16 days from the prior menses beginning the pituitary produces a surge of a different hormone called luteinizing hormone or LH. The LH surge stimulates the ovary to release the egg, and for the remaining cells of the follicle the egg was released from to further develop into what is called the corpus luteum. The corpus luteum produces progesterone. The progesterone causes the lining of the uterus to mature and become able to support a fertilized egg.

If the egg is fertilized and successfully implants in the uterus the developing embryo produces a hormone called human chorionic gonadotropin which stimulates the corpus luteum to continue to develop and produce progesterone. If no pregnancy occurs, without the HCG feedback the ovary stops producing progesterone after about 12-14 days, and without the progesterone support the lining of the uterus starts to be shed, resulting in the monthly menstrual bleeding. This bleeding lasts for 3-8 days in most women. On about the first day of bleeding, the hypothalamus recognizes that the progesterone levels have dropped, and that no pregnancy has occurred, and it again stimulates the pituitary to release FSH, starting a new cycle.

This cycle repeats itself until a pregnancy develops, the ovary fails to respond in menopause, or artificial hormone intervention like birth control pills are used.