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

The uterus 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.

A woman has a fallopian tube and ovary on either side of the uterus. 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. Many girls get their first period around the age of 12 or 13 years. However, the age of onset varies considerably and girls may begin menstruating any time from age 8 to 16.

Both the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists recommend that adolescent girls see their primary care physician for preventive healthcare visits to discuss their reproductive health, and help establish an environment where they can feel comfortable discussing health concerns. Preventive health visits during adolescence can offer an opportunity for physicians to educate teen girls and their parents on what comprises normal puberty, menarche and the menstrual cycle.

During the first few years of menstruation, a girl’s period may be irregular. Eventually it should develop into 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 women. However, it can last 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.

Women 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).

Menstruation continues monthly for most 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 no longer ovulate (produce eggs) regularly and eventually women stop menstruating. After one year without a period, a woman is considered to have entered menopause. A woman in menopause cannot become pregnant. Surgical removal of the ovaries also causes a woman to reach menopause immediately, regardless of age. Many women experience several years of transition to menopause (perimenopause), when the menstrual periods may become inconsistent.