U.S. girls are developing breasts at a younger age compared to years past, and obesity appears to explain a large share of the shift, a new study suggests.
Researchers found that between 2004 and 2011, American girls typically started developing breasts around the age of 9. And those who were overweight or obese started sooner -- usually when they were about 8 years old.
The numbers are concerning, the researchers said -- especially since the typical age at breast development is younger now than it was in a similar study from 1997. The main reason: Girls are heavier now than they were in the '90s.
"This is another manifestation of America's high body-mass index," said lead researcher Dr. Frank Biro, of Cincinnati Children's Hospital Medical Center. Body-mass index (BMI) is a measure of body fat based on a ratio of height to weight.
The findings, reported online Nov. 4 and in the December print issue of the journal Pediatrics, add to evidence that American children are hitting puberty earlier than in decades past. The rising tide of childhood obesity has been suspected as a major cause, but the new study gives more hard data to support the idea.
Biro said, however, that excess pounds do not seem to be the full explanation. And it's possible that other factors -- such as diet or chemicals in the environment -- play a role.
Why should people worry that puberty is coming sooner now than in years past? There is a concern when young kids look older than they are, and are possibly treated that way, Biro said.
Studies have found that girls who mature early are more likely to be influenced by older friends, start having sex sooner and have more problems with low self-esteem and depression. "Just because you're developing more quickly physically doesn't mean you're maturing emotionally or socially," Biro said.
Plus, early puberty has been tied to long-term health risks. For women, an earlier start to menstruation has been linked to a heightened risk of breast cancer. It's not clear why, but some researchers suspect that greater lifetime exposure to estrogen might be one reason.
Biro said earlier puberty also has been tied to increased risks of high blood pressure, heart disease and diabetes in adulthood. It's hard, though, to know whether earlier puberty is to blame since obese kids tend to start puberty earlier, and obese children often become obese adults, he said.
Dr. Patricia Vuguin, a pediatric endocrinologist at the Steven and Alexandra Cohen Children's Medical Center in New Hyde Park, N.Y., said it's not known if it's the earlier development or the obesity itself that causes the increased risk of those conditions.
But it is clear that childhood obesity has consequences, said Vuguin, who was not involved in the study. "For parents, the message is to pay more attention to a healthy diet and exercise even in a child's early years," she said.
The findings are based on 1,200 girls from three U.S. cities who were followed between 2004 and 2011. Black girls started developing breasts around age 8, while Hispanic, white and Asian girls typically started at age 9.
When Biro's team compared their findings with the 1997 study, they found that white girls were clearly maturing faster in recent years. Twenty-one percent had started developing breasts before age 9, versus 11 percent in the earlier study, for example.
Black girls seemed to be developing earlier too: 22 percent started before age 8, versus 15 percent in 1997. But that difference was not significant in statistical terms, Biro said.
"What seems to be happening is that white girls are 'catching up' with black girls," he said.
From the data, it appeared that increasing body weight accounted for much of the difference between the two studies, Biro said. But excess pounds did not tell the whole story, he said.
It's possible that other factors are at work, Biro said, including various environmental chemicals that can disrupt hormone activity, such as certain pesticides and plasticizers. Lack of exercise and childhood diets that are low in fiber and high in meat and dairy also have been suspected of contributing to earlier puberty. But none of those suspicions has yet been proven.
Vuguin said research like this is important for doctors because they need to keep evaluating what "normal" development is. There is a difference between relatively earlier puberty and what doctors call "precocious puberty" -- which can have consequences such as stunted growth because the bones stop maturing earlier than normal.
Traditionally, Vuguin said, precocious puberty has been defined as signs of puberty -- including breast development -- before the age of 8 in girls. Doctors may use hormonal medications to treat it.
"But if you have, for example, an obese African-American girl who is developing breasts at the age of 7, that might be 'normal' now," Vuguin said. "I think there is starting to be a shift in how we're looking at this: Should we treat it, or let it be? It's complicated."
The American Academy of Pediatrics has more on development during puberty.
A version of this story originally appeared on iVillage.