Doctors and parents were stunned when research published more than a decade ago found American girls were beginning puberty at much younger ages, some as early as 7. A new study released Sunday suggests the average age at which puberty begins may still be falling for white and Latina girls.

According to the paper, which appears in the journal Pediatrics, almost one quarter of African American girls have reached a stage of breast development marking the onset of puberty by age 7, as had almost 15 percent of Latina girls and more than 10 percent of white girls.

Those percentages are significantly higher than in 1997, when a landmark study first reported that girls were beginning puberty much younger than they had in the mid-20th century. In that study, the rate of girls who had begun puberty at age 7 was, on average, 5 percent for whites, compared with 10.4 percent in the new study.

“In 1997, people said, ‘That can’t be right; there must be something wrong with the study,’ ” said Dr. Frank M. Biro, director of adolescent medicine at Cincinnati Children’s Hospital Medical Center and the lead author of the new study. “But the average age is going down even further.”

Biro’s study included 1,238 girls ages 6 to 8 who lived in one of three regions: Cincinnati; East Harlem, N.Y.; or San Francisco. Puberty was determined by two examiners who worked independently to assess the girls’ breast development. By age 8, 27 percent of the girls had begun puberty: 18.3 percent of whites, 42.9 percent of blacks and 30.9 percent of Latinas.

Compared with data from the 1997 study, the age at which puberty begins did not fall for African American girls, although they still mature at younger ages than white or Latina girls. It’s not clear why there was no change for black girls. “Perhaps black girls have approached a biologic minimum,” Biro said.

There are numerous potential explanations for why puberty is starting earlier. Chief among them is the increase in average body weight among children over the last three decades, Lee said. Excess body weight, especially body fat, is thought to increase the blood levels of estrogens that promote breast development. Earlier studies, including one by Lee, have linked early puberty to higher body mass index as far back as the toddler years.

But other studies suggest that body fat may not be the only cause. A Danish study released last year in the journal Pediatrics found puberty occurring earlier in children regardless of body mass index at age 7. Factors may include a diet that is increasingly high in sugar and fat, declining physical activity and exposure to endocrine disrupters, chemicals in the environment that act on hormones.

“Kids today are exposed to plastic much more than they were 10 or 20 years ago,” Lee said.