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The Duration of the Menstrual Cycle

What is considered a normal menstrual cycle can vary greatly from person to person, just like one person’s cycle can vary from month to month. Irregularities in the menstrual cycle are common and, in most cases, no cause for concern. However, they sometimes signal health problems or significant life changes, such as pregnancy or the onset of menopause.

The menstrual cycle is a series of natural changes, governed by hormones that prepare a woman’s body for the possibility of pregnancy—the development and release of an egg and the thickening of the uterine lining to provide nutrients for the egg, should it become fertilized. If ovulation occurs and the egg is not fertilized, the uterine lining is shed through the vagina as the menstrual period, lasting two to seven days on average.

The length of the menstrual cycle is calculated from the first day of menstruation until the first day of the next menstrual period. Although 28 days is said to be the average length, a cycle of 21 to 35 days is considered to be within normal range.



Studies have found that only about 13 percent of women have a 28-day cycle.

It is normal for young women to have irregular menstrual cycles when they first begin to menstruate. Many women find that their cycles stabilize within two years, but some have irregular periods for their entire lives. It is unusual for women to experience a difference of more than 4 days between the length of their shortest and longest cycles, but a difference of up to 8 days is still considered regular. Variations of 8–20 days between shortest and longest cycles is considered irregular, but not abnormal. Variations of 21 days or longer are considered very irregular.

What are the four phases of the menstrual cycle? What are the possible causes of polymenorrhea and oligomenorrhea? Find out more about the Duration of the Menstrual Cycle here.

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