Menstruation, also called the menstrual cycle, prepares the sexually mature, female body for pregnancy each month. Menstruation is monthly bleeding that begins in girls around 12 years of age. Because bleeding associated with the menstrual cycle happens regularly or periodically, it also is referred to as a menstrual period or, more simply, as a "period." Some girls get their first menstrual period as young as 9 years of age, and others do not begin menstruating until 15 years of age.

Menstrual periods usually stop during pregnancy. In some cases, breastfeeding also suppresses menstruation, but this varies from woman to woman. Certain health conditions can affect menstrual periods or cause periods to stop. Women generally stop having menstrual periods around the age of 50. The cessation of menstruation is called menopause.

The Menstrual Cycle

Menstruation involves the organs of the female reproductive system. These organs, which are located in the lower abdomen or pelvic region, include the following:

  • The ovaries are two small organs that secrete hormones (e.g., estrogen) and produce and store female sex cells (eggs, ova). Each month, one egg matures and is released from one of the ovaries. Generally, the ovaries alternate from one month to the next in releasing an egg.
  • Once the egg is released by the ovary, it travels from the ovary to the uterus through the fallopian tubes. There are two fallopian tubes, one for each ovary.
  • The uterus, also called the womb, is the hollow, muscular organ in which a fertilized egg implants and develops until birth, first into an embryo and then, after the eighth week, into a fetus.
  • The cervix is the lower part of the uterus. It is a narrow opening between the uterus and the vagina.
  • The vagina, also called the birth canal, leads from the uterus to the outside of the body.

During each menstrual cycle, the lining of the uterus thickens in preparation for pregnancy. This lining, which is called the endometrium, is composed of blood and tissue. The endometrium provides a surface for a fertilized egg to attach and develop if pregnancy occurs.

As the endometrium thickens, an egg in one of the ovaries begins to mature or ripen. When the egg is released by the ovary, it enters one of the fallopian tubes. If a male sex cell (called a sperm) fertilizes the egg, it may implant in the lining of the uterus, resulting in pregnancy. If the egg is not fertilized, it is shed through the vagina with blood and tissue from the endometrium in menstruation.

Day 1 of the menstrual cycle generally is considered the first day of bleeding. Bleeding associated with menstruation usually lasts 2–7 days. Menstrual bleeding can vary from light to moderate to heavy. The volume of menstrual blood varies from woman to woman and from cycle to cycle. Normal blood loss ranges from 44 mL to 80 mL.

After each menstrual period, the cycle of preparing the body for pregnancy begins again. The length of the full menstrual cycle is measured from day 1 of bleeding to day 1 of bleeding in the next menstrual period. This cycle usually is about 28 days, but it can vary in length from 25 to 36 days. Adolescent girls and older women may have menstrual cycles that are less regular (i.e., change often) and can be as many as 45 days apart.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 15 Nov 2008

Last Modified: 25 Sep 2015