Menstrual cycle

The menstrual cycle is the repeating sequence of events that, from puberty to menopause, readies the uterus to receive an embryo. By convention, day one is the first day of bleeding, and each cycle lasts until the next period begins. It averages about 28 days, but adult cycles anywhere from roughly 24 to 38 days are considered normal, and the length often varies from one cycle to the next.

Hormones from the hypothalamus, pituitary gland and ovaries run the cycle. In the follicular phase, follicle-stimulating hormone (FSH) helps a group of ovarian follicles grow, and the leading follicle makes estrogen, which rebuilds the endometrium, the inner lining of the uterus. Once estrogen is high enough, it sets off a surge of luteinizing hormone (LH) that releases an egg, usually about two weeks before the next period. The emptied follicle turns into the corpus luteum, whose progesterone readies the lining for implantation during the luteal phase.

If no embryo implants, the corpus luteum breaks down, progesterone falls and the lining is shed as menstruation, starting the cycle again. Students meet the cycle as a classic example of both negative and positive hormonal feedback.