The findings suggest earlier development than what was reported in a 1997 study and show a worrying pattern, say the study's authors, led by Dr. Frank Biro of Cincinnati Children's Hospital Medical Center. Girls who hit puberty earlier are more likely to engage in risky behavior, Biro's team notes, and might be at a higher risk for breast cancer, than their peers who develop later.

This could represent a real trend, Dr. Joyce Lee, a pediatric endocrinologist at the University of Michigan who was not involved with the new research, told Reuters Health.

Doctors are unsure of what could be causing girls to develop at a younger age, but rising obesity rates may be to blame, they say.

In a study published today in Pediatrics, Biro's team examined about 1,200 girls aged 7 and 8 in Cincinnati, New York and San Francisco. Researchers, as well as the girls' doctors and nurses, used a standard measure of breast development to determine which girls had started puberty.

Compared to the 1997 findings from girls across the U.S., girls in the current study - especially white girls - were more developed at a younger age. As previous research has shown, there were also large differences in development based on race.

At age 7, approximately 10 percent of white girls and 23 percent of black girls had started developing breasts - compared to 5 percent of white girls and 15 percent of black girls in 1997, the authors write.

Among 8-year-olds in the study, 18 percent of white girls and 43 percent of black girls had entered puberty - an increase from around 11 percent of white girls from 1997, but the same as black girls in that year.

This study and another published today in Pediatrics suggest that being overweight, both as a young child and growing up, makes girls more likely to enter puberty earlier. In the second study, Dr. Mildred Maisonet from Emory University's Rollins School of Public Health and her colleagues observed that gaining weight quickly in infancy - a predictor of later obesity - was linked to early puberty in girls in Great Britain.

Biro's team found that girls with a higher body mass index (BMI) - a ratio of weight and height - at age 7 and 8 were more likely to be developed than their thinner peers.

Those authors warn that their study population, although diverse, doesn't necessarily represent what's happening in all U.S. girls. But they are continuing to follow the girls in the study to see when the rest of them hit puberty, and what other factors might be related to their rate of development.

Biro thinks that rising rates of obesity could be a major reason why girls seem to be developing faster than they did even 13 years ago. We're on the opposite side of an increase in BMI that has been seen in this country and in other countries, he told Reuters Health.

Researchers know that heavier girls are more likely to enter puberty early, Lee, of the University of Michigan, said. That could be because overweight people have more of a hormone known to be linked to development - but it could also be a matter of the actual nutrients that girls get from their diet, she said.

Lee and Biro said doctors are worried about both the psychological and physical health of girls who hit puberty at a young age.

Studies have shown that girls who develop early are more at risk for depression and often start having sex earlier than girls who develop later.

For the 11-year old that looks like she's 15 or 16, adults are going to interact with her like she's 15 or 16, but so are her peers, Biro said. Girls who develop early look physically older, he said. It doesn't mean that they're psychologically or socially more mature.

In addition, women who spend more of their lives menstruating are at a higher risk for breast cancer - which, depending on when they hit menopause, could be a worry for girls who develop early.

Biro said that there are things families can do to minimize the possible risk of early puberty in young daughters, including eating more fruits and vegetables and eating together as a family.

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