Most girls begin their menstrual cycles just before reaching their teens, when their bodies are growing into early womanhood. The average age of menarche (or the beginning of menstruation) in the United States is 12.5 years. Physical and sexual maturation is happening at earlier ages now than 15 or 20 years ago, and if a girl is on the large size for her age and has been developing other signs of puberty over the past year or so, she is probably on an appropriate though early end of the normal time line.
With every milestone or biologic happening, there is a range of normal. Think about height. Plenty of people are 5'6" and are normal, but so are the folks who are 6'2'' and those who are 5'3". Same with weight. Weighing 105 lbs is normal, so is weighing 140 lbs. It does matter, though, how old you are, what racial or ethnic group you come from, and what your parents' height and weight were. It also matters if—all of a sudden—your weight or height changed or if you attained it at an appropriate pace.
The same is true of menarche. Because this biologic occurrence has a range, some girls will start menarche on the early side, and some on the later end. The pattern is determined generally by several factors: mother's age at menarche, race/ethnic group, and weight (or BMI).
When a girl is beginning to menstruate at this young age, it will be worthwhile checking in with your pediatrician, who will do a targeted history about your family pattern, look at the girl's weight curve and her other growth parameters (including other signs of early sexual development), and decide if any endocrine testing is necessary. Your pediatrician will also talk to you about the emotional impact of being an early maturer and will give you tips for how to handle the situation. If a girl is the first in her class to have this experience, her friends will shortly be joining her. She may even be able to help them understand that menarche comes for all girls sooner or later and is an important and normal part of growing up.
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