The beginning of menstruation is a major marker in the transition from girl to woman. The age of menarche (men-are-kee) — when a girl first begins to menstruate — varies, depending on many factors.
Some factors are biological; for instance, body fat must be about one quarter of a girl’s total weight in order for her to menstruate. Diet, weight, race, environment, and family history also affect when menstruation begins.
In the United States, the average age of girls’ first period has fallen over the last century from around age 16 to around age 12, with some girls beginning to develop breasts as early as age seven. (A visit with a healthcare provider is recommended if signs of puberty do not appear by age 13, or if a girl’s period does not start by age 16.)
The reasons girls are maturing earlier are not completely clear. Some pediatricians and other medical experts are convinced that childhood exposure to plastics, pesticides, and other environmental endocrine disruptors plays a role. (Endocrine disruptors are chemicals that often act like estrogen and interfere with genetic or hormonal signals, causing changes to the body’s finely tuned hormonal system.) Other possible causes include better nutrition than in the past, obesity, inactivity, premature birth, and formula feeding.
A major concern about early puberty in girls is that it increases the odds of developing breast cancer later in life, since longer exposure to sex hormones is a risk factor for breast cancer. Once a girl starts to menstruate, her estrogen levels rise and don’t drop off until menopause. All other factors being equal, the more years of menstruation equals the more years of exposure to higher levels of estrogen, and the higher her risk of breast cancer.
Not all of us have periods. Some medical conditions, such as being born without a vagina or uterus, affect reproductive development.