There's yet another reason to worry about the obesity epidemic among America's kids: Extra weight may be sending U.S. girls into puberty earlier than ever.
Researchers have found that girls with higher body mass index, a ratio of height and weight, may start developing breasts more than a year before their thinner friends—perhaps as early as second grade.
The change is spawning a whole new market of child-sized sanitary pads—decorated with hearts and stars—and deodorants aimed at 8- to 10-year-olds, according to a new study and an editorial published Monday in the journal Pediatrics.
"The girls who are obese are clearly maturing earlier," said Dr. Frank Biro, a pediatrics professor at Cincinnati Children's Hospital Medical Center. "BMI is, we found, the biggest single factor for the onset of puberty."
In addition, white girls are maturing about four months earlier than in a landmark 1997 study that shocked parents with the news that their daughters who played with My Little Pony could be entering puberty.
Biro's team followed more than 1,200 girls ages 6 to 8 in three cities—San Francisco, Cincinnati and New York—from 2004 to 2011, carefully documenting their BMI and their maturation process.
(Read more: I can keep my health-care plan? Not true)
On average, girls with BMIs below the 50th percentile started developing breasts at about age 10, while those in higher categories—the 85th and 95th percentiles—began developing as early as age 8.5.
Why is this happening? Although they're not certain, scientists including Biro say that extra weight might trigger the body to believe it has enough energy and other resources to kick off puberty—even when the child in question is only 8.
Obesity can't be called the cause of early puberty, researchers emphasized. But it is one of several factors—along with inactivity, and chemicals in food and water—that may be affecting the hormones that trigger sexual maturation. Other recent studies have shown that puberty may be starting earlier in boys, too.
About 17 percent of U.S. children and teens, or some 12.5 million kids, are obese, according to the Centers for Disease Control and Prevention.
Biro's work is more evidence that the entire spectrum of puberty in the U.S. may be shifting downward, said Dr. Neerav "Nick" Desai, an adolescent medicine specialist at Vanderbilt University, who was not involved in the study.
(Read more: Millions can get free health policies)
The notion that U.S. kids were entering puberty earlier was once controversial, but studies like this show there's no doubt. "The numbers in this study are very precise," Desai said. "It's more precise than any other."
One thing that those precise numbers show is that while puberty in all girls started at a little older than age 9—about two years younger than in earlier decades—it varies among races. On average, black girls start breast development at age 8.8, Hispanic girls start at age 9.3 and white and Asian girls start at about age 9.7. Overall, though, the trend is toward earlier onset.
The proportion of first-grade black girls with breast development tripled in the past 15 years. Now, 18 percent of black first-graders have breast buds, a figure that rises to 38 percent by third grade. For white girls, it's about 4 percent of first-graders, rising to 21 percent, noted an editorial by Marcia Herman-Giddens. She's the professor of maternal and child health at the University of North Carolina at Chapel Hill whose research first raised startling questions about early puberty.
"Each individual girl is exposed to multiple factors in today's environment, many not present decades ago, that pay potentially influence her pubertal onset," she wrote.
The new study analyzed only breast development, not the start of menstruation, which will be the topic of further research, Biro said. He'll also be looked at the influence of chemicals such as bisphenol A (BPA), which can disrupt certain hormones, to see how they may influence early development.
Starting puberty early can lead to a host of problems for girls, including risk for lower self-esteem, depression, early sexual activity and being negatively influenced by older peers.
Parents of girls who may be heavy have many reasons to help their girls slim down, but they shouldn't focus on weight control in order to delay puberty, Biro said.
"Parents of these early maturing kids have to be more watchful," he said. "But I don't want to have a nation of patients with eating disorders. We need to figure out what are healthy weights for our kids. We want them to be comfortable with their bodies."
—By JoNel Aleccia, NBC News