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There's no conclusive evidence on what could be causing girls to develop early signs of puberty, but obesity and environmental exposure to chemicals are among factors being explored. ((Patricia Ariss/CBC))

Some American girls are developing breasts as young as age seven, researchers have found.

The study, published in Monday's online issue of the journal Pediatrics, looked at more than 1,200 girls age six to eight in New York, Ohio and California. It compared the age when the girls showed early signs of puberty against the results of a similar study from 13 years ago.

Over that time period, the age at which girls began showing early signs of puberty decreased among all races:

  • Among seven-year-olds, about 10.4 per cent of white girls, 23.4 per cent of black girls and almost 15 per cent of Hispanic girls had started developing breasts.
  • Among eight-year-olds, 18.3 per cent of white girls, nearly 43 per cent of black girls and just under 31 per cent of Hispanic girls showed evidence of breast development.

In comparison, about 5.0 per cent of seven-year-old white girls showed signs of breast development in the 1997 study, as did 15.4 per cent of black girls. No comparison data was available for Hispanic girls.

"The proportion of girls who had breast development at ages seven and eight years, particularly among white girls, is greater than that reported from studies of girls who were born 10 to 30 years earlier," the study's authors concluded.

Cause unknown

The researchers have no conclusive evidence about what could be causing girls to develop teenage bodies earlier.

Obesity may be a factor since girls who developed breasts early tended to have a higher body-mass index than those who didn't.

The study's authors said they are concerned that girls who develop breasts earlier might not be able to cope with the social and emotional consequences, such as having to deal with advances from males and the hormonal changes that come with puberty.

"She's just not that mature yet to know what to do about these changes," agreed Dr. Peter Nieman, a pediatrician in Calgary who is also seeing the trend in his practice. "People that will think she is older than she is so that could cause some self-esteem issues."

Pollutant levels to be probed

Two different trained health professionals examined the girls two separate times between 2004 and 2006 to distinguish between breast tissue and fatty tissue, the researchers said.

Pollutants that mimic the female hormone estrogen might also be contributing to early puberty, said study author Dr. Frank Biro of Cincinnati Children's Hospital.

"Whether [they be in] food that they've eaten, or products that are used for personal care products, as well as products that could be used at their homes," Biro said.

The researchers are beginning studies to determine whether environmental exposures to chemicals could be contributing to earlier puberty among girls.

It is thought that a combination of genetics, environment and individual factors like weight cause puberty to begin.

Regardless of the cause, parents can help to limit the risk for their daughters, said Dr. David Lau, an endocrinologist at the University of Calgary's School of Medicine.

"Make sure they achieve good health in terms of not becoming overweight, physically being active and eat healthy foods and try to avoid exposure to environmental toxins and chemicals," Lau suggested.

Previous research on puberty, specifically the menstrual cycle, has indicated that girls who start menstruating at age 11 or younger have an increased life-long risk of breast cancer.