Menstruation is the monthly shedding of tissue and blood that is the most visible part of the menstrual cycle. The menstrual cycle involves all of the woman’s reproductive organs. It is an indicator of good health for reproductive-aged girls and women and also prepares the body for pregnancy.

Girls usually begin menstruating around the age of 12 or 13, but can begin menstruating as young as age 9. In some cases, girls may begin menstruation before age 9 (precocious puberty). Although parents may feel uncomfortable about the prospect of discussing menstruation with their daughters, this conversation is crucial before the girl begins having her monthly cycle. The American Academy of Pediatrics (AAP) recommends discussing puberty, including menstruation, with both boys and girls around age 8 or 9.

The uterus (womb) is a fist-sized, hollow organ in the lower abdomen that can expand to accommodate a growing fetus. The bottom of the uterus (cervix) meets the top portion of the vagina, the tube that leads outside the body. Menstrual blood leaves the body through the vagina. The lining of the uterus (endometrium) includes blood vessels, glands and tissue that grow and thicken to support a fetus during pregnancy. When pregnancy does not occur, the endometrium breaks down and is shed each month during menstruation.

Each fallopian tube extends from the top of the uterus to an ovary, the organs that produce the woman’s eggs. Every month, a mature egg leaves one of the ovaries and travels through the fallopian tube to the uterus. If the egg is fertilized by a sperm, it can implant in the endometrium of the uterus and develop into an embryo. If the egg is not fertilized, it disintegrates and is absorbed into tissue or leaves the body with the menstrual flow.

The first menstrual period (menarche) is part of the sexual development girls go through during puberty. In the early 1900s, girls were likely to have their first menstrual periods around the age of 14 or 15 years. However, improved nutrition and other factors have resulted in girls getting their periods earlier, typically around the age of 12 or 13, but sometimes as early as age 8 or as late as age 16. Many factors influence when a girl first begins to menstruate, including genetics and lifestyle choices, such as excessive exercise (which can deprive a female’s body of the calories, vitamins and minerals it needs to menstruate).

In most cases, girls begin to menstruate about two years after their breasts start to grow. In other cases, this pattern is reversed. A girl may experience a thick, white discharge from her vagina for several months before her first period.

During the first few years of menstruation, a girl’s period may be irregular (anovular). Some girls may only have three or four periods a year. Within about two years of first menstruating, girls should settle down to a more regular monthly cycle. The average menstrual cycle lasts about 28 days, but may range from 21 to 35 days. Normal cycle length varies among women and for a particular woman at different times in life.

The menstrual period usually lasts about five days each month for most girls. However, it can last anywhere from two to eight days. The flow of menstrual blood may be heavy for several days and then light. It may vary in volume and length from month to month. The use of birth control pills may make menstrual periods lighter and shorter.

Girls use various products to absorb menstrual flow. Sanitary napkins (pads) are worn inside the panties. Tampons are inserted into the vagina. Both are made of cotton or another absorbent material. Pads or tampons should be changed frequently (every few hours). When girls first start menstruating, they may prefer to use pads instead of tampons. Girls who use tampons should not find them uncomfortable if the tampon is properly inserted.

Menstruation continues monthly for most girls and women in their reproductive years. Pregnancy interrupts menstruation because the endometrium must nourish the growing fetus. Menstruation resumes after childbirth, sometimes taking several months to return to a normal cycle. Breastfeeding may also affect the return of menstruation.

By about age 50, women’s estrogen levels begin to decrease. The ovaries can no longer ovulate (produce eggs) regularly and eventually women stop menstruating. One year without a period is called menopause. A woman in menopause cannot become pregnant. Surgical removal of the ovaries may also cause a woman to reach menopause immediately, regardless of age. Many women experience several years of transition to menopause (called perimenopause), where menstrual periods may become less frequent.