Why Are Girls Hitting Puberty Earlier?

We've heard the reports that girls are hitting puberty sooner than ever before. After all, if you can believe it, child-size pantyliners and kids' deodorant are even real products. Now, a new study has found that puberty is kicking in earlier than ever, with most girls starting to mature before age 10 — and as soon as at age eight and a half for girls with especially high Body Mass Indexes (BMI).

“The girls who are obese are clearly maturing earlier,” said Dr. Frank Biro, a pediatrics professor at Cincinnati Children’s Hospital Medical Center. “BMI is, we found, the biggest single factor for the onset of puberty.”

Biro followed 1,200 6- to 8-year-old girls in three American cities — Cincinnati, San Francisco, and New York — to track different indications of puberty, as well as their BMIs. Girls in the 85th and higher percentiles of BMI began developing breasts around age 8.5, while girls with BMIs below the 50th percentile didn't have the need for bras until age 10. Scientists speculate that the extra weight heavier girls carry could trick the body into thinking it had enough calorie resources to kick off pubescent changes.

The study comes at a time when obesity rates among America's children actually seems to be curbing — though it's doing so slower for girls than for boys. However, doctors are quick to point out that obesity isn't a 'cause' of early puberty so much as a factor contributing to it. Inactivity and chemicals in food and water also may also be triggering the process.

"Each individual girl is exposed to multiple factors in today's environment, many not present decades ago, that may potentially influence her pubertal onset," wrote Marcia Herman-Giddens, a professor of maternal and pediatric health at the University of Carolina-Chapel Hill, Biro's study in an editorial accompanying the study.

The majority of girls began showing signs of Tanner II development (i.e. the first physical manifestations of puberty, as noted on the Tanner scale) before age 10: For 10-year-olds, 75 percent show signs of breast development and 57 percent have started growing pubic hair. But by 11, the numbers swing upward: 91 percent of girls have started growing breasts, and 86 percent see hair down there — and these are averages, encompassing all BMIs.

When the Tanner Scale was first developed by British pediatrician James Tanner in conjunction with his mid-20th-century study of puberty, the average age for girls to begin Stage II was 11. Now, almost all girls hit its checkmarks of breast and pubic hair development by that age.

The downward shift indicated in Biro's study illustrates what has become obvious within the pediatric circles. The Tanner stages, long used as points-of-reference for pubescent development, have even already been modified to reflect earlier average starts to puberty. (The scale now carries an addendum saying that for African-American girls, puberty begins up to two years earlier, and for Caucasian girls one year earlier than it used to.)

The earlier start times for different ethnic groups are also one case where Biro's numbers are clear: African-American girls start developing breasts at 8.8 years old; Hispanic girls begin on average at 9.3; and Caucasian and Asian girls begin around 9.7.

“Parents of these early maturing kids have to be more watchful,” Biro said. “But I don’t want to have a nation of patients with eating disorders. We need to figure out what are healthy weights for our kids. We want them to be comfortable with their bodies.”