Researchers looked at more than 1,200 girls in the Bay Area, Cincinnati and New York City. They found girls with a higher body mass index, a ratio of height and weight, started developing breasts before their thinner friends.
Results show the biggest age difference in Caucasian girls, who entered puberty at the average age of 9.7 years old, four months earlier previously reported.
African American girls started puberty at 8.8 years old, which was not different from before. The average age for Hispanic girls was 9.3 years and 9.7 years for Asian girls.
San Rafael's Zero Breast Cancer organization says the concern is that early puberty has been linked to a higher risk of breast cancer.
The report is published online in the journal Pediatrics.