American girls are hitting puberty earlier than ever a change that puts them at higher risk for behavioral problems as adolescents and breast cancer as adults, a new study shows.

About 15% of 1,239 girls studied showed the beginnings of breast development at age 7, according to an article in today's Pediatrics. One in 10 white girls, twice as many as in a 1997 study, showed breast growth by that age, as did 23% of black girls and 15% of Hispanic girls.

The median age of breast development fell from 10.9 years in 1991 to 9.9 in 2006, according to a Danish study published in Pediatrics last year.

The new study doesn't explain why girls are developing earlier, but it did find heavier girls with a higher body-mass index were more likely than others to begin puberty early, says pediatrician Frank Biro, director of adolescent medicine at Cincinnati Children's Hospital Medical Center.

A third of children are now overweight, and the early puberty trend could be related to the obesity epidemic, says Marcia Herman-Giddens of the University of North Carolina-Chapel Hill. A growing number of researchers also are concerned about hormone-disrupting chemicals in the environment. Animal studies suggest that many environmental
toxins can affect the age of puberty, although scientists aren't yet sure exactly how they affect people.

Suspect chemicals include pesticides used in farms and lawns, flame retardants found in furniture and electronics, and bisphenol A, or BPA, an estrogen-like ingredient found in plastic bottles, the linings of metal food and beverage cans, Biro says. He notes that researchers are collecting blood and urine samples from girls and will be able to analyze their exposure to toxins.

Animal and human studies suggest certain chemicals may affect male sexual development.

The herbicide atrazine, for example, has been shown to chemically "castrate" some male frogs and turn others into females able to lay eggs, according to a March study in Proceedings of the National Academy of Sciences.

And a 2008 study found that baby boys are more likely to have genital changes, such as undescended testicles and smaller penises, if they were exposed before birth to high levels of phthalates, hormone-disrupting chemicals found in medical tubing, vinyl and other products.

The chemical industry says these chemicals are safe and have never been definitively proven to harm humans.

Hitting puberty at a young age can be confusing and distressing, Herman-Giddens says.

It also increases the odds that girls will develop low self-esteem, eating disorders and depression. Girls who hit puberty sooner are more likely to attempt suicide and to have earlier sexual activity. As adults, these women are at greater risk for breast and endometrial cancers, possibly because they have a longer lifetime exposure to estrogen.

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