You can read how lead author Dr Frank Biro, director of adolescent medicine at Cincinnati Children's Hospital Medical Center, and colleagues reached these conclusions in a paper published online in Pediatrics on 9 August.
Biro told the press that it wasn't clear what was causing these girls to reach puberty earlier, but they were looking at several potential factors, including genes and environment, and how they might interact.
One of the reasons perhaps to be concerned about this trend is that previous studies have shown a link between earlier start of menarche (when a girl gets her first period) and higher risk of breast cancer.
For the study, Biro and colleagues examined data on 1,239 girls between the ages of 6 and 8 from East Harlem in New York; the greater Cincinnati metropolitan area; and the San Francisco Bay area in California.
The girls were recruited by Breast Cancer and the Environment Research Centers set up in 2003 by the National Institute of Environmental Health Science and the National Cancer Institute.
The data came from physical examinations of the girls and interviews with caregivers.
The researchers used a well-established set of guidelines for deciding onset of puberty, including a five-stage system of classifying breast development called Tanner Breast Stages.
They found that a higher proportion of girls, and white girls in particular, were developing breasts earlier at age 7 and 8 than found in studies done 10 and 30 years ago.
The results also showed that:
- 10.4% of white, 23.4% of black non-Hispanic, and 14.9% of Hispanic girls had reached breast stage 2 or higher at age 7.
- At age 8, these figures were: 18.3%, 42.9%, and 30.9%, respectively.
- The proportion of girls who had reached breast stage 2 also varied by site and BMI (body mass index), with more obese girls reaching puberty earlier.
However, if this is a real trend, it could have important implications for public and individual health.
Biro told Reuter's Health that girls who enter puberty earlier are not only at higher risk of breast cancer later in life, but also more likely to engage in risky behavior.
Girls who enter puberty earlier are psychologically younger than they look, but could be regarded as older, and be expected to behave accordingly, which could affect their mental and emotional development. Studies show that girls who enter puberty earlier are also at higher risk for depression.
Biro said that rising obesity rates could be a reason why girls are entering puberty earlier and like everyone, they should be encouraged to follow a healthy diet high in fruits and vegetables.
"Pubertal Assessment Method and Baseline Characteristics in a Mixed Longitudinal Study of Girls."
Biro, Frank M., Galvez, Maida P., Greenspan, Louise C., Succop, Paul A., Vangeepuram, Nita, Pinney, Susan M., Teitelbaum, Susan, Windham, Gayle C., Kushi, Lawrence H., Wolff, Mary S.
Pediatrics 2010 0: peds.2009-3079
Published online August 9, 2010, doi:10.1542/peds.2009-3079
Additional sources: Cincinnati Children's Hospital, Reuter's Health.
Written by: Catharine Paddock, PhD