( Health.com ) -- Girls in the United States are entering puberty at earlier ages than they have in the past , a new study reports . More than 10 percent of white 7-year-old girls in the study , which was conducted in the mid-2000s , had reached a stage of breast development marking the start of puberty , compared to just 5 percent in a similar study conducted in the early 1990s . Black and Hispanic girls continue to mature faster than white girls , on average . Nearly one-quarter of black girls and 15 percent of Hispanic girls had entered puberty by age 7 , according to the new study , which appears in the journal Pediatrics . But the trend toward earlier puberty is not as pronounced among blacks as it is among whites , the researchers say . Although the rate of early puberty among black girls in the study ( 23 percent ) was higher than that observed in the early 1990s ( 15 percent ) , the increase was not statistically significant . Health.com : Is it baby fat or obesity ? `` White girls are catching up , '' says Dr. Frank Biro , M.D. , the lead author of the study and the director of adolescent medicine at Cincinnati Children 's Hospital . Experts are n't sure what 's behind the increase in earlier puberty , but it 's likely due to a combination of factors , including the childhood obesity epidemic and substances in the environment . Early puberty in girls is a growing public health concern because studies have shown that girls who start puberty earlier are more likely to develop breast and uterine cancer later in life . The National Institutes of Health funded the study as part of a larger investigation into the environmental factors that contribute to breast cancer risk . Video : Girls becoming women sooner . `` Breast cancer is such a common problem , so if we can find some of the things that make it more likely , we could improve screening of those early developers , '' says Dr. Susan Nickel , M.D. , an assistant professor of pediatrics at the Texas A & M Health Science Center College of Medicine , in Temple . ( Nickel was not involved in the new research . ) Cancer risk is n't the only concern surrounding early puberty . Early development in girls has been linked with poor self-esteem , eating disorders , and depression , as well as cigarette and alcohol use and earlier sexual activity . `` If an 11- or 12-year-old girl looks like she 's 16 , people will interact with her as if she were 16 , '' Biro says . `` Early maturation increases the rate of risk-taking behaviors and lowers academic performance . It does n't mean it 's going to happen , but it could . '' Health.com : Plastic baby bottles and BPA : Health risk or hysteria ? Between 2004 and 2006 , Biro and his co-authors assessed the breast development of more than 1,200 girls ages 6 to 8 in three major U.S. cities ( New York , Cincinnati , and San Francisco ) . The stage of development in which a girl 's breasts begin to `` bud '' is considered the onset of puberty , not her first menstrual cycle . The average age of the first period has declined as well , says Nickel . `` Girls used to get their first menstrual period at 14 or 15 , '' she says , but now the average is closer to 12 . Health.com : The big three hormones . Regardless of their race , girls with a higher body mass index ( BMI ) -- a ratio of height to weight -- tend to mature sooner , the researchers found . This finding , which has been reported in other studies , suggests that soaring obesity rates among children may be contributing to the rates of early puberty . But it 's not yet clear why girls -- and especially white girls -- are starting to physically mature at younger ages . One theory is that excess body fat affects the levels of hormones that trigger puberty . Health.com : 4 things that mess with your hormones . Chemicals in the environment -- most notably bisphenol-A ( BPA ) , which is found in many hard plastic products -- may affect hormones as well , says Dr. Gary Berkovitz , M.D. , a professor of pediatric endocrinology at the University of Miami 's Miller School of Medicine . A girl 's experience in infancy and even in the womb may also contribute to earlier puberty , according to a second study that appears in the same issue of Pediatrics . Health.com : Make play areas greener and safer for kids . In that study , which included about 4,000 girls in the United Kingdom , researchers from the Centers for Disease Control and Prevention found that mothers who had their first period before age 12 , smoked during pregnancy , and were pregnant for the first time had daughters who entered puberty sooner than other girls . Above-average weight gain during infancy was also associated with earlier puberty , the study found . Copyright Health Magazine 2011 .
Experts are n't sure what 's behind the increase in earlier puberty . Studies showed girls who started puberty earlier at greater risk for certain cancers . Doctor : Chemicals such as bisphenol-A ( BPA ) may affect hormones .