Boston Women's Health Book Collective answered:
Girls experience first periods in a variety of ways, as do women experiencing last menstrual cycles. Biology contributes to these variations, but so do the place, time, and culture in which we live. During puberty, we make the transition from childhood to physical maturity. In women, puberty is characterized by growth of the breasts and the pubic and armpit (axillary) hair, and a growth spurt that results in increased height and weight. Bone size and strength stop increasing around puberty, but bone mass continues to grow through the twenties. The reproductive process is regulated by hormones, which are chemicals in the bloodstream and brain that relay messages from one part of the body to another.
The levels of sex hormones are low during childhood, increase tremendously during the reproductive years, and then become lower and balance differently after menopause. The changes women experience around menarche and menopause (and during their entire menstrual lives) are thought to be caused primarily by the changing levels of hormones. Ovulation and menstruation start near the end of puberty, on average at about twelve and a half, though any age from nine to eighteen is normal. The age of menarche varies depending on many factors. (Menarche, pronounced men-ar-kee, is when girls get their first period.) Some factors are biological; for instance, a girl needs her body fat to be about one quarter of her total weight to menstruate. To sustain regular cycles, women also need to eat a balance of fat, carbohydrates (sugars and starches), and protein. Some factors are due to our environments. Women in different cultures may enter puberty at different times.
Girls living in the same country may have different average ages of menarche depending on factors such as diet, weight, race, environment, and family history. During the reproductive years, cycles of hormone rhythms determine the timing of ovulation and menstruation. This cycle, the menstrual cycle, regulates our fertility, allowing for the possibility of pregnancy a number of days every month. Many women also experience more outward signs of this rhythm—changing emotions, changes in their breasts, variation in foods they enjoy eating at different times over a month. Menstruation and ovulation continue until age fifty (on average), but anytime between forty and fifty-five is normal. When periods stop, menopause has occurred.Find out more about this book: Our Bodies, Ourselves: A New Edition for a New EraGirls experience first periods in a variety of ways, as do women experiencing last menstrual cycles. Biology contributes to these variations, but so do the place, time, and culture in which we live. During puberty, we make the transition from... More