Girls in the United States are entering puberty at earlier ages than they have in the past, a new study reports.

More than 10 percent of white 7-year-old girls in the study, which was conducted in the mid-2000s, had reached a stage of breast development marking the start of puberty, compared to just 5 percent in a similar study conducted in the early 1990s.

Black and Hispanic girls continue to mature faster than white girls, on average. Nearly one-quarter of black girls and 15 percent of Hispanic girls had entered puberty by age 7, according to the new study, which appears in the journal Pediatrics.

But the trend toward earlier puberty is not as pronounced among blacks as it is among whites, the researchers say. Although the rate of early puberty among black girls in the study (23 percent) was higher than that observed in the early 1990s (15 percent), the increase was not statistically significant.

‘White girls are catching up,’ says Frank Biro, M.D., the lead author of the study and the director of adolescent medicine at Cincinnati Children’s Hospital.

Experts aren’t sure what’s behind the increase in earlier puberty, but it’s likely due to a combination of factors, including the childhood obesity epidemic […]