Menstruation is the discharge of blood and tissue that occurs each month as part of a woman’s menstrual cycle. This cycle is controlled by hormones produced in both the brain and ovaries and prepares the reproductive organs for pregnancy. The lining of the uterus (endometrium) thickens in response to the hormones. If pregnancy does not occur, the lining begins to break down and discharges from the body through the vagina as the menstrual period.

Menstruation usually begins when a girl is about 12 or 13 years old (puberty) and continues until menopause. These are considered a woman’s reproductive years and indicate the woman is physically able to become pregnant. Menstrual periods usually occur once a month and last for several days during each month. A missed menstrual period is frequently the first sign that pregnancy has occurred. The uterine lining is not shed, but rather will grow during pregnancy and develop into the placenta (a temporary organ that provides nourishment for the developing embryo). Missed periods may also indicate the presence of other gynecological conditions, such as hormonal or endocrine disorders.

The blood flow of menstruation may vary from month to month. Women typically use sanitary napkins (pads) or tampons to absorb menstrual flow. Before and during menstruation, women may experience some mild to moderate cramps, water retention and irritability. Some of these symptoms may be treated with over-the-counter analgesic (painkiller) medication. However, symptoms that do not respond to painkillers or that worsen over time may indicate more serious conditions, such as endometriosis.

Hormones control the functions of the menstrual cycle. They signal the ovaries to develop and release the eggs. Hormones also signal the endometrium to initially begin to grow and develop, and to break down when menstruation occurs. As a woman ages and her hormone levels begin to decrease, the menstrual cycle eventually ends (menopause). Menopause typically occurs during a woman’s early 50s.