The first menstrual period, which occurs at an average age of 12.8 years for girls in the United States
The age at which girls begin to menstruate varies widely and may be influenced by a number of factors, most notably the percentage of body fat. Anywhere from 10.8 years old to 14.6 years old is considered normal. In the months preceding menarche, many girls experience a small amount of clear or whitish vaginal discharge, an indication that the first period will soon take place. Family history plays an important role in determining when a girl's periods will start. For example, mothers who started to menstruate early will often have daughters who also start early. In most girls, menarche occurs at about two years after the beginning of breast development and one year after the growth spurt. For the first year or so after menarche, a girl's ovaries do not release eggs into the fallopian tubes.
Bell, Ruth, et al. Changing Bodies, Changing Lives. New York: Vintage, 1988.
American College of Obstetricians and Gynecologists
Address: 409 12th Street
Washington, DC 20024 (Brochures about menstruation and other topics)
—Gail B. Slap, M.D. University of Pennsylvania School of Medicine