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From NPR News, this is ALL THINGS CONSIDERED. I'm Melissa Block.

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And I'm Michele Norris.

Girls as young as 7 are showing signs of sexual development, and it's happening faster than previous findings suggested.

As NPR's Brenda Wilson reports, scientists aren't sure why.

BRENDA WILSON: The rate of 7-year-old, white girls who began developing breasts early doubled in the last decade - 10 percent of 7-year-old, white girls compared to 5 percent in a landmark study conducted more than a decade ago.

In the new study in the journal Pediatrics, researchers followed 1,200 girls in Cincinnati, the San Francisco Bay Area and East Harlem, New York.

Dr. Frank Biro, a pediatrician and the director of the Cincinnati Children's Hospital, was the lead author.

FRANK BIRO: We found, to our surprise, that the rates of pubertal maturation were much higher than reported among the white participants. The black participants were essentially identical, the rates of maturation.

WILSON: In other words, there wasn't much change over the last decade for African-American girls, but they are already three times as likely to start puberty early. Hispanics are in between. It's easy to pick these girls out in a crowd.

BIRO: You can just look in the classroom, they were a half a head taller than their classmates. If you're a pediatrician, you can sort of see early breast development, just at a casual glance at the girls. You could sort of see these changes.

WILSON: The first person to call attention this problem a decade ago was Marcia Herman-Giddens at the University of North Carolina. She conducted a landmark study in 1997. She says there may be some criticism of the new study because it only looked at girls in three urban areas.

MARCIA HERMAN: However, the data are, of course, quite concerning. And it certainly is a wake-up call and means that a larger study needs to be done.

WILSON: It's important to find out what's going on, she says, because studies have shown that girls who begin their menstrual periods early are at a slightly higher risk of breast cancer. She says there's reason to believe there is a similar link to early breast development.

HERMAN: So if girls are starting breast development two or three years earlier than they were 40 or 50 years ago, of course, that gives them that many more years of exposure to estrogen.

WILSON: And long-term exposure to estrogen is linked to an increase in the risk of breast cancer.

Dr. Paul Kaplowitz is an endocrinologist at National Children's Hospital in Washington, D.C. He sees young girls with early breast development and says not every case is a cause for concern.

PAUL KAPLOWITZ: What I think is hard to tell from a study like this is how many of these girls have just a little breast development, which stabilizes and doesn't progress in the next six to 12 months, and how many of these girls have early breast development - that if you saw them six months later, you'd say, oh, my goodness, this girl is going through puberty, she's going to start having periods early, and we need to intervene.

WILSON: Scientists don't know what triggers the early onset of puberty. Chemicals in the environment, the increasingly inactive life of children, and the growing epidemic of obesity among U.S. children are considered culprits. Study author Dr. Frank Biro says a change in lifestyle wouldn't hurt.

BIRO: I think that we could all stand living a little greener. That would probably be a healthier approach to life. So I think that we could try to eat more of the fruits and vegetables, eating together as a family.

WILSON: The researchers will continue to follow the girls in this study through puberty.

Brenda Wilson, NPR News.

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