LOS ANGELES -- 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 still might be falling for white and Latina girls.

According to the paper, which appears in the journal Pediatrics, almost one-quarter of black 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 other words, the average age of puberty onset still appears to be in flux.

"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.

Even for white and Latina girls, it is too early to declare that puberty age is still falling, said Dr. Joyce Lee, assistant professor of pediatric endocrinology at C.S. Mott Children's Hospital at the University of Michigan, who was not involved in the study.

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 past three decades, Lee said. Excess body weight, especially body fat, is thought to increase the blood levels of estrogens that promote breast development.

But other studies suggest that body fat might 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 might 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.

Biro said that his study would continue to follow the girls' development and that blood and urine samples are being collected to look at biomarkers that reflect potential environmental exposures.

Early development in girls is not inconsequential. Studies have linked it to various health risks. Early maturation in a large population of girls also may affect future breast cancer rates.

Parents may be able to influence the timing of puberty, Biro said, through such lifestyle decisions as encouraging a healthful diet and physical activity, and avoiding lotions, shampoos and other products for children and babies that contain phthalates, which are known endocrine disrupters.