Menstruation, or a menstrual period, refers to the monthly process in which the uterus sheds blood and tissue to prepare for pregnancy.

Not having or missing a menstrual period is called amenorrhea. This condition is divided into two types:

  • Primary amenorrhea—when an adolescent female has not yet begun menstruation by around age 16 years
    • Most females begin menstruating between the ages 9-18, but age 12 is the average.
  • Secondary amenorrhea—when a woman who has previously menstruated misses three or more periods in a row

Menstrual Flow
Menstrual Flow
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The most common cause of secondary amenorrhea is pregnancy. In nonpregnant women, it may be due to a variety of factors.