SAN FRANCISCO — Girls are starting puberty at younger ages — a full year earlier than previously reported in some cases — and the main factor associated with early breast development is obesity, according to a new long-term study being released Monday.

Researchers in the San Francisco Bay Area, New York and Cincinnati have been following the sexual maturation of more than 1,200 girls — since 2004. At the start of the study, the girls were between 6 and 8 years old. Breast development at younger ages is a concern among physicians and parents because early menstruation, which can accompany it, has been linked to higher risk of breast cancer.

Additional research has also found other potentially detrimental health, environmental and social impacts of precocious adolescence. Health experts say obesity is the biggest factor in the early onset of puberty, but the next step is to understand the factors that lead to obesity.

“It may not just be as simple as calories in, and calories out,” said Janice Barlow, executive director of Zero Breast Cancer, a San Rafael, Calif., group. “There may be other factors in the environment that may be fueling the obesity epidemic.”

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Barlow said the discoveries may help breast-cancer research go beyond detection and treatment.

The study, which was to be published Monday online in the medical journal Pediatrics, found some differences among ethnic groups. White participants, for instance, showed early stages of breast development at 9.7 years, about a year earlier than was noted in a landmark 1997 study of more than 17,000 girls nationwide. The average age for black girls did not change significantly in the two studies but remained at 8.8 years old. The median age for Latinas in the latest study was 9.3 years, and 9.7 years for Asian girls. Neither demographic was represented in the 1997 study.

Monday’s study showed that girls with high pediatric body mass index — a measurement used to assess a child’s physical growth in relation to other kids of the same age — started developing breasts earlier. White girls with very high BMIs — in the 85th percentile — developed almost two years earlier than normal-weight girls.

“Now that we have this information, we are in the position of going into the things that could be causing it (early sexual maturing),” said Dr. Louise Greenspan, a study co-author and pediatric endocrinologist at Kaiser Sam Francisco.

The researchers are now looking at environmental and psychological factors behind the shift including diet and exercise, exposure to chemicals that may have an impact on hormones and social stressors like problems in the home or at school.

The study is considered the largest and longest research project to follow girls throughout the stages of puberty.

Next, the researchers plan to release a series of studies that look at different stages of puberty, including the start of menstruation.

This study focused on breast development because budding breasts, more than pubic hair growth or other changes, is considered the true onset of puberty in girls.