Although the average age for the beginning of menstruation is between 11 and 13, a few girls develop breasts before they are eight or have their first period by nine. This condition is called precocious puberty.

About 90 percent of girls who menstruate early have "true" precocious puberty, that is, their reproductive system functions exactly like that of an adult. These youngsters secrete the hormones necessary for menstruation. They ovulate, are fertile, and have secondary sex characteristics. Most become short women because their higher-than-normal estrogen level stops their growth at an early age. The underlying cause is an abnormality in the brain.

Girls with "pseudo" precocious puberty do have increased estrogen levels, but do not produce the other reproductive hormones as adults do. Because of the increased estrogen, they develop secondary sex characteristics, but they are not fertile. Causes range from ovarian or adrenal tumors that produce estrogen, to an underactive thyroid, or use of certian cosmetics and estrogen-containing foods and medications.

To make a diagnosis, the doctor will check estrogen levels, inquire about birth injuries or a family history of brain disease and perform various other tests. Treatment focuses on medications which prevent the release of reproductive hormones such as GnRH. Injections of the birth-control drug Depo-Provera reduce the amount of estrogen, stop menstruation, decrease breast development, and allow growth to develop normally. However, its long-term effects may be serious. Girls who menstruate early may also need psychological counseling.