Amenorrhea refers to the absence of a menstrual cycle. Primary amenorrhea occurs if an adolescent female has not yet begun menstruation by around age 16 or so. Secondary amenorrhea describes a condition in which a woman who has previously menstruated on a regular basis misses three or more periods in a row. In the United States, amenorrhea affects between 2-5% of women.
Most females begin menstruating between the ages of 9-18, but age 12 is the average. Menstruation, or a menstrual period, refers to the monthly process in which the uterus sheds blood and tissue in preparation for pregnancy.
If a female has not started her menstrual period by age 16, a doctor should be contacted to determine the cause and prescribe any treatment that may be necessary.