Modern puberty

Puberty involves three stages: breast development, pubic hair growth and, finally, menstruation. Because the final event is typically the most memorable for women, it has been the one most scientifically documented in studies based on self-reported memories. The first 100 years that medical records were kept on the age of onset of menstruation saw continuous drops. Between about 1850 and 1950 in Europe, the average age of a girl's first period dropped from about 17 to about 13. (The U.S. doesn't have good data earlier than the 20th century, though trends were probably similar, says Steingraber, who prepared the August 2007 report after examining hundreds of studies on potential dietary, lifestyle and environmental causes of early puberty.)

Much of that decline probably has to do with better nutrition and public health improvements that reduced the spread of infectious diseases. "Better diet, closed sewer systems, deep burial of the dead," Steingraber says. "By the beginning of the 20th century, those things were in place."

Adequate food and good health signal the brain that it's safe to reproduce, according to theories of evolutionary biology. "We're healthier and we weigh more," says Dr. Francine Kaufman, head of the center for diabetes and endocrinology at Childrens Hospital. "In some ways, puberty is a luxury."

With the brain picking up these signals, the hormonal parade can begin, first with the release from the hypothalamus of gonadotropin-releasing hormone, which sends other hormones from the pituitary gland through the bloodstream to the ovaries. The ovaries gear up production of a form of estrogen called estradiol, which initiates breast development -- the first step in puberty.

A second signaling pathway stimulates the adrenal gland to begin androgen production, which results in pubic hair. The final stage of puberty is the beginning of monthly periods.

But the first two events are happening significantly earlier in the lives of today's girls than they did in the lives of their mothers and grandmothers. The age of first menstruation has dropped too, at a rate of about one month per decade for the last 30 years, according to a January 2003 study in Pediatrics. Today, the U.S. average for first period is 12.5 for white girls, 12.06 for black girls and 12.09 for Latinas.