When Dr. Frank Biro began enrolling subjects in a national, long-term study of early on-set puberty, he went into the Greater Cincinnati schools to find girls who might participate.
Biro decided to visit first and second grade classrooms with the hopes of enrolling girls who had not yet started the hormonal changes that lead to maturity.
Instead what he found were many girls - more than reported in earlier studies - who looked like they were already starting puberty.
"Especially in the second grade, I walked into the classroom - I mean these girls are half a head taller than everybody else - You can see from across the room that they're in puberty. Not only are they taller but you can see that they have breast development. And I said, 'ewe, I'm not going to be able to capture what happened before they hit puberty, because they are already in it," Biro says.
Biro is a physician in the division of adolescent medicine at Cincinnati Children's Hospital Medical Center and the lead investigator in a national study published last month in the journal Pediatrics.
Along with colleagues in the San Francisco Bay Area and New York City, Biro's study included more than 1,200 girls between the ages of 6 and 8 years old at the time they were enrolled. Trained clinicians examined the girls at regular intervals for seven years, assessing breast development and body mass index - or BMI.
When all the data was compiled and analyzed, a clear conclusion emerged: "Girls with a higher body mass index hit puberty earlier," Biro says.
The researchers examined timing of breast development and BMI and found that as the girls' BMI increased over the 50th percentile, the likelihood of earlier breast development also increased.
"When you take a look at the white participants in the study, they were clearly maturing earlier than previously reported. But they're still not maturing as early as most of the African American girls," Biro says.
In fact, black girls had the lowest median age at which breasts begin to develop, at 8.8 years old.
Next were Hispanic girls, at 9.3 years old, followed by white and Asian girls at 9.7 years old.
Dr. Sumana Narasimhan is a pediatric endocrinolgist at University Hospitals in Cleveland. The link to obesity, she says, is clear. On average, she sees four patients a week experiencing the symptoms of early on-set puberty.
Aside from the health risks of obesity, she says, the children also face the challenges that come with maturing early. First, their bodies grow quickly and, for many girls, that means they will stop growing earlier.
"So if you allow puberty to occur earlier, the girl may look really tall but she's going to end up really short. And, so, it's a stature issue. The second thing that is important to families is of course, the psychological maturity. And how is, say, a first grader or a second grader, you know, going to deal with body changes?" Narasimhan says.
Early maturity can lead to emotional struggles, or even depression, she says.
In some cases, specialists can recommend hormones that young girls can take to slow maturation. But in most cases, simply knowing what is happening can help the family cope.
Dr. Biro at Cincinnati Childrens' Hospital says as the research into early onset puberty continues, the medical community is debating whether a "new normal" is coming into play.
"I think the biggest debate is what is abnormally early and what is not abnormally early. Our data would suggest that there's a substantial proportion of girls who are maturing early if one does use the previous definitions of early maturation," Biro says.
But Biro says so many American girls - more than 25 percent - are experiencing puberty sooner because of obesity, it's difficult to say what is normal and what is early.