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What is menstruation?

Menstruation (pronounced men-stroo-EY-shuhn) is normal vaginal bleeding that occurs as part of a woman's monthly menstrual cycle. Menstruation occurs between menarche (pronounced muh-NAHR-kee), a girl's first period, and menopause.1 Menstrual blood flows from the uterus and passes out of the body through the vagina (see diagram). The average menstruation time in normally menstruating women is about 5 days .2 In the United States, most girls start menstruating shortly after age 12.3

What is the menstrual cycle?

Menstruation is one part of the menstrual cycle, in which female hormones prepare the uterus to support a pregnancy. The average menstrual cycle is 28 days, but this varies between females and from month to month. In young females it can range from 21 to 35 days.3 The menstrual cycle and pregnancy are described below.

Day 1

The first day of bleeding is considered the first day of the menstrual cycle. After bleeding ends, usually around day 5, levels of the hormone estrogen begin to rise. The rise in estrogen causes the lining of the uterus to thicken as it prepares to hold a fertilized egg. At the same time, the changes in hormone levels cause an egg to start to mature in one of the ovaries.


Around day 12, the egg leaves the ovary in a process called ovulation (pronounced ov-yu-LAY-shuhn). Ovulation can occur anywhere between 10 and 21 days after the first day of a woman's menstrual cycle. A woman can tell when she has begun ovulating using several methods, including at-home tests that measure hormone levels in the urine and by keeping track of her body temperature.

If a pregnancy does not occur, the lining of the uterus is shed during menstruation.

This diagram illustrates how the endometrium builds up and breaks down during the menstrual cycle. The endometrium is thickest halfway through the 28-day cycle. Then, if there is no pregnancy, it breaks down. This breakdown causes the bleeding of the menstrual phase.

What happens when a pregnancy occurs?


After ovulation, the egg moves down the fallopian tube. The sperm can fertilize the egg at this point. Fertilization of the egg is also called conception (pronounced kuhn-SEP-shuhn). After the sperm is ejaculated into the vagina, it moves into the cervix and through the uterus into the fallopian tube. Sperm can live up to 5 days in a woman's body. If fertilization occurs, the newly formed embryo travels back through the fallopian tube into the uterus, where it implants in the wall of the uterus. If fertilization does not occur, the egg breaks apart, and the uterine wall is lost in the form of menstrual bleeding.


The embryo must successfully implant in the thickened wall of the uterus for the pregnancy to continue. The embryo first attaches to the wall of the uterus (usually 5 or 6 days after ovulation) and then it becomes more firmly implanted between 6 and 12 days after ovulation. Implantation causes a release of hCG-a hormone that signals the body to change to support the pregnancy. This hormone is what a pregnancy test detects.

  1. Sweet, M. G., Schmidt-Dalton, T. A., Weiss, P. M., & Madsen, K. P. (2012). Evaluation and management of abnormal uterine bleeding in premenopausal women. American Family Physician, 85, 35-43. 
  2. Dasharathy, S. S., Mumford, S. L., Pollack, A. Z., Perkins, N. J., Mattison, D. R., Wactawski-Wende, J., & Schisterman, E. F. (2012). Menstrual bleeding patterns among regularly menstruating women. American Journal of Epidemiology, 175, 536-545. 
  3. McDowell, M. A., Brody, D. J., Hughes, J.P. (2007). Has age at menarche changed? Results from the National Health and Nutrition Examination Survey (NHANES) 1999-2004. Journal of Adolescent Health, 40, 227-231.


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