Between 2004 and 2006, Dr. Biro and his co-authors assessed the breast development of more than 1,200 girls ages 6 to 8 in three major U.S. cities (New York, Cincinnati, and San Francisco). The stage of development in which a girl’s breasts begin to “bud” is considered the onset of puberty, not her first menstrual cycle.

The average age of the first period has declined as well, says Dr. Nickel. “Girls used to get their first menstrual period at 14 or 15,” she says, but now the average is closer to 12.

Regardless of their race, girls with a higher body mass index (BMI)—a ratio of height to weight—tend to mature sooner, the researchers found. This finding, which has been reported in other studies, suggests that soaring obesity rates among children may be contributing to the rates of early puberty.

But it’s not yet clear why girls—and especially white girls—are starting to physically mature at younger ages. One theory is that excess body fat affects the levels of hormones that trigger puberty.

Chemicals in the environment—most notably bisphenol-A (BPA), which is found in many hard plastic products—may affect hormones as well, says Gary Berkovitz, MD, a professor of pediatric endocrinology at the University of Miami’s Miller School of Medicine.

A girl’s experience in infancy and even in the womb may also contribute to earlier puberty, according to a second study that appears in the same issue of Pediatrics.

In that study, which included about 4,000 girls in the U.K., researchers from the Centers for Disease Control and Prevention found that mothers who had their first period before age 12, smoked during pregnancy, and were pregnant for the first time had daughters who entered puberty sooner than other girls.

Above-average weight gain during infancy was also associated with earlier puberty, the study found.