Body Mass Index (BMI) is a strong indicator of early onset of puberty in girls than race or ethnicity, according to a new study led by Cincinnati Children's Hospital researchers.

The researchers arrived at the conclusion after observing the stages of puberty in more than 1,200 girls in San Francisco, New York and Cincinnati who were aged 6 to 8 at the start of the study in 2004.

They found that girls with higher BMI (a ratio of height and weight) start developing breasts sooner (as early as second grade) than their thinner peers.

"The impact of earlier maturation in girls has important clinical implications involving psychosocial and biologic outcomes," said Frank Biro, MD, lead investigator and a physician in the Division of Adolescent Medicine at Cincinnati Children's Hospital Medical Center in a press release. "The current study suggests clinicians may need to redefine the ages for both early and late maturation in girls."

Girls who experience puberty at an earlier age suffer from lower confidence, higher rates of depression, norm-breaking behaviors and lower academic achievement. It also leads to hypertension and several cancers - including breast, ovarian and endometrial cancer.

"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."

The study also found that black girls began puberty at a median age of 8.8 years, while white, non-Hispanic girls experienced breast development at a median age of 9.7 years, earlier than previously believed.  

The research has been published in the journal, Pediatrics.