JACKSONVILLE, Fla. -

A new study, published by the journal "Pediatrics" shows that girls are starting puberty much earlier than in years past.

"It's a trend that we've been noticing for a while now, that the girls are being referred earlier because they're hitting puberty earlier," said Nemours' Dr. Robert Olney.

The most common age for girls to start puberty is usually around 12 or 13 years old, but a new study shows that girls as young as 6 and 7 are starting puberty early.

Researchers say obesity could be to blame for the early changes.

"About 18 percent of the African American girls were starting puberty between 6 and 7 years old, which is second grade," said Dr. Olney.

In the December issue of the journal "Pediatrics," researchers found that between 2004 and 2011, African American girls started developing breasts at 8 years old; among Caucasian girls, the age was about 9 and a half. The age was several months younger than it was in similar studies done in the 90s.

"The striking thing about the article was the association with obesity. The girls who were overweight and obese were starting puberty strikingly earlier than we had realized," said Olney.

Olney said nutrition and exercise can play a role, but there are other factors that also could contribute to early puberty.

"The study is looking at a number of environmental factors. They're looking at some chemicals, some of the things that we call 'endocrine disruptors' that may be playing a role, but we won't have those results for several more years," said Olney. "There are some associations, we know for breast cancer in particular, the earlier you start puberty, you have a higher risk of developing breast cancer some time in your life. There's a lot of psycho-social issues as well. A lot of girls have a higher risk of depression and a higher risk of low self-esteem. They tend to get a little more attention -- unwanted attention -- because they look older than they are."