Most girls have their first menstrual period between the ages of nine and 16. For the first year or two, periods will probably be irregular; it may not come at the same time every month. A girl can even have her first period and not have another one for months. In most cases, periods become regular (about once a month) within two years of the first period.

Generally, girls can expect a first period about two to three years after the first signs of breast development. There are several things that can affect the onset of menstruation. One thing is genetics. The age when your mother had her first period may be a clue. Your weight may also have an effect-either too thin or too heavy. Athletic girls also tend to start menstruating later.

In the final stages of puberty, young women reach physical, emotional and sexual maturity. They grow to their full height, breasts reach their full size, and girls develop a regular pattern of menstruation.