Many girls in the United States are showing the beginning of breast development as young as 7 and 8, a new study reports.

Researchers said the earlier sexual development is closely linked to the rising rates of childhood obesity.

More than 1,200 girls ages 6 to 8 from Cincinnati, East Harlem, N.Y. and San Francisco were examined for the presence of breast tissue. Among 7-year-old white girls, more than 10 percent had started developing breasts, according to the study published in the September issue of Pediatrics.

Black and Hispanic girls showed even faster maturity rate with 23.4 percent of black girls and almost 15 percent of Hispanic girls who had started developing breasts.

Among 8-year-olds, 18.3 percent of white girls, about 43 percent of black girls and just under 31 percent of Hispanic girls showed evidence of breast development.

The study was a part of a larger investigation into the environmental factors that contribute to breast cancer risk.

Experts found the results alarming because early puberty is associated with a higher risk of breast and uterine cancer throughout the life span, according to Dr. Frank Biro, director of adolescent medicine at Cincinnati's Children's Hospital.

Many factors could be driving the earlier maturation, but it’s likely due to a combination of childhood obesity epidemic and chemical substances in the environment.

Researchers reported that girls who developed breasts early are more likely to have a higher body-mass index (BMI) than those who didn’t. Although more research needs to be done on the linked between high BMIs and early puberty, fat cells produce the hormone leptin which is involved in the onset of pubertal maturation, Biro said.

Biro and his colleagues are conducting further research on the link between environmental exposure to chemicals and early puberty in girls.