(USA TODAY) -- The age at which puberty starts in some girls has continued to drop, finds the latest study to suggest obesity may be a key trigger.
The start of puberty (determined by a specific stage of breast development) was at a median age of 9.7 years old in white, non-Hispanic girls, 4 months earlier than in a landmark 1997 study that first documented early puberty in U.S. girls. The report is in December's Pediatrics, published online today.
The new study tracked 1,200 girls (enrolled at ages 6 to 8) over seven years. It found blacks continue to develop earlier than whites, reaching puberty at a median age of 8.8, vs. 9.3 years for Hispanics and 9.7 for Asians.
But blacks matured at similar ages in the 1997 study; it was not clear why there was no change for blacks. For Hispanics and Asians, data was insufficient to know if puberty age changed.
While previous studies found that race as well as body mass index (BMI) played a role in predicting the onset of puberty, "BMI supplants race as the No. 1 factor" in the new analysis, says lead author Frank Biro, director of adolescent medicine at Cincinnati Children's Hospital.
Girls with higher BMIs in all race and ethnic groups were progressively more likely to have started puberty.
Although this and other studies implicate weight as a "prime driver" of early puberty, Biro says the question remains, "Why are they more obese?" In addition to increased calorie consumption and less physical activity, researchers are looking at a number of other possible explanations, such as exposure to chemicals in the environment that may trigger hormonal changes, he says.