Girls are entering puberty at younger ages now than a couple decades ago, a new study suggests.
The study researchers aren't sure what's causing the early puberty, but they suggest multiple factors, including genetics, environmental chemicals and an increase in body fat and obesity.
While breast tissue growth and other physical signs of puberty are not in themselves red flags, they could be linked with health risks, including breast cancer, said study researcher Dr. Frank Biro of Cincinnati Children's Hospital Medical Center in Ohio.
"I think it pretty much confirms what people have been saying for quite a while," said Dr. Paul Kaplowitz, chief of endocrinology at Children's National Medical Center in Washington, D.C., who was not involved in the current study.
Kaplowitz was referring to a study published in 1997 showing a downward trend in age of puberty onset for girls, along with a more recent study out of Denmark last year showing the same thing.
The new research is detailed this week in the journal Pediatrics.
The new findings are based on examinations of more than 1,200 girls ages 6 through 8 from three U.S. cities. Trained female professionals examined the girls for signs of puberty onset, including conducting breast palpations that looked specifically for breast tissue, not fat. One argument against the 1997 findings and others like it has been that perhaps girls who are overweight just have more fat tissue around their breasts and haven't actually begun puberty.
The results of the new study showed that 10.4 percent of white non-Hispanic girls had begun puberty by age 7, compared with 23.4 percent of black girls and 14.9 percent of Hispanic girls. Among 8-year-olds, puberty had begun in 18.3 percent of white non-Hispanic girls, 42.9 percent of black girls and 30.9 percent of Hispanic girls.
White non-Hispanic girls showed the largest drop in age of puberty onset between the 1997 study and the new findings. The white non-Hispanics in the new study were nearly twice as likely to have reached puberty by ages 7 and 8 than were girls of that age in the 1997 study, Biro said. That's a significant increase between the early 1990s, when those girls were examined, and 2004 to 2006, when Biro's study was conducted.
"It certainly was more than we had anticipated," Biro told LiveScience, referring to the change compared with the 1997 results. "So this was a bit of a surprise to us."
However, Kaplowitz cautioned, "I wouldn't go so far as to say that the study shows the trend for early puberty has intensified." There could be other reasons for the difference between the two study results.
Causes of early development
Part of the problem could be childhood obesity. One of the big differences between the two studies, Biro said, was the body mass index (BMI; a measure of a person's weight in relation to their height), with girls in the recent study having higher numbers than in the past. "Overall, the girls in the United States now have a higher body mass index than they did 20 or 30 years ago. It's not just obesity but the whole BMI has gone up," he said.
Other studies have demonstrated a link between weight and age at which girls begin puberty. [3 Simple Steps Can Cut Childhood Obesity]
Environmental factors might also play a role. Some household products and pesticides contain so-called endocrine disruptors, which are synthetic chemicals that, when absorbed by the body, can mimic or block hormones and disrupt normal functions, such as growth and maturation.
"I don't think we can rule out chemicals in the environment, but nobody has provided persuasive evidence," Kaplowitz said.
Biro's study was initiated as part of a breast cancer research program, because a younger age of menarche (first menstrual period) has been linked with a higher risk for developing breast cancer.
An analysis of past studies showed that for premenopausal and postmenopausal women, the risk of breast cancer was decreased by 9 percent and 4 percent, respectively, for each year that menarche was delayed. That analysis was published in 2002 in the British Journal of Cancer.
"For an individual girl it's not going to impact her risk of breast cancer much," Biro said during a phone interview. "But it could have an impact from a population basis."
However, though girls seem to be starting puberty earlier, the age of their first period hasn't changed as much over the past 40 years. Perhaps, Kaplowitz speculates, the girls who are showing an earlier puberty onset are not developing as fast after that, taking longer to reach menarche.
Beside health risks, the physical body changes could have other impacts on certain girls and their parents.
Parents "don't want their kids to grow up too soon," Kaplowitz said. Parents also think their daughters might have a tough time physically adjusting to the changes so early.
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Jeanna served as editor-in-chief of Live Science. Previously, she was an assistant editor at Scholastic's Science World magazine. Jeanna has an English degree from Salisbury University, a master's degree in biogeochemistry and environmental sciences from the University of Maryland, and a graduate science journalism degree from New York University. She has worked as a biologist in Florida, where she monitored wetlands and did field surveys for endangered species. She also received an ocean sciences journalism fellowship from Woods Hole Oceanographic Institution.