Girls are entering puberty at a younger age than ever before and obesity may be a reason why, according to a new study published Monday.

Researchers found that girls with higher body mass index, the ratio of weight and height, may develop breasts at a year younger before girls with lower body mass index (BMI). The study, published in the journal Pediatrics, says girls nowadays may start to enter puberty as early as second grade.

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

A key finding in the study was girls with higher BMIs are more likely to start puberty early, regardless of race or ethnicity.

The study tracked 1,200 girls, whose ages at the beginning of the study ranged from six to eight over the course of seven years. Researchers carefully documented the girls’ body mass index.

It was found that blacks develop earlier than whites girls, at the median age of 8.8 versus 9.7 years old. The median age for Hispanics is 9.3 years and 9.7 for Asians.

The study also marks a drop in the age when white, non-Hispanic girls start puberty. The median age of when white, non-Hispanic girls begin developing is now four months earlier than what was found in a 1997 study that first revealed early puberty in U.S. girls.

The most recent study shows that blacks mature at similar ages from the 1997 study. There was not enough data to determine whether there was a change for Hispanic and Asian girls, according to USA Today.

“Each individual girl is exposed to multiple factors in today’s environment, many not present decades ago, that pay potentially influence her pubertal onset,” Marcia Herman-Giddens, professor of maternal and child health at the University of North Carolina at Chapel Hill, said.

The new study analyzed only breast development and not the start of menstruation, which needs to be researched further, Biro said.

NBC News reports that early puberty can lead to several problems for girls, such as low self-esteem, depression and early sexual activity.

Sources: USA Today, NBC News