Girls are developing breasts at a younger age and an upward trend in childhood obesity seems to be playing a major role.
Researchers say they have found African American girls on average started getting breasts just before they turned nine.
Among white girls the average age was about nine and a half - a few months earlier than in the 1990s.
The findings "confirm an ongoing downward trend in pubertal timing among US girls," said Dr Anders Juul, who heads the Department of Growth and Reproduction at Copenhagen's Rigshospitalet in Denmark.
"It's been worrying for the US as well as the rest of the world," Juul said.
The data come from a long-term study of
more than 1 200 girls in and around San Francisco, Cincinnati and New York
Girls were enrolled in the study between ages six and eight and they were studied at semi-annual or annual visits. At each visit, staff assessed breast development using Tanner stages, which measure how far along a young person is in puberty.
African American girls first showed signs of breast development at eight years 10 months on average. That compared to nine years four months among Hispanic girls and nine years eight months among white and Asian girls.
Puberty for white girls hit about four months earlier than in a 1997 study that also measured breast development.
That study concluded girls were entering puberty earlier than in the past. Heavier girls tended to start developing at younger ages. Rising obesity rates seem to be a "prime driver" behind breast development starting earlier, said Dr Frank Biro.
Juul's own work has suggested obesity isn't behind earlier breast development among Danish girls.
Researchers said how much exercise girls get, diabetes precursors and chemicals in the environment that can mimic hormones may all play a role in pushing up the onset of puberty.
Biro said one of their challenges was pinpointing chemicals that could also play a role.He works in the adolescent medicine division at Cincinnati Children's Hospital Medical Centre in Ohio.
Where girls live, meat and dairy in their
diets and family stress have also been tied to earlier development, Marcia
Herman-Giddens wrote in a commentary on the report.
She studies maternal and child health at the University of North Carolina at Chapel Hill.
The study didn't look at when girls started getting their periods, or at other measures of puberty.
The researchers said
maturing at a younger age could come with long-term risks for some girls. For
instance, those who hit puberty earlier could be at higher risk of breast and
other cancers because their bodies spend more years making and being exposed to
Way they look
They also tend to start having sex or using
drugs and alcohol at younger ages and are more likely to become depressed or
develop low self-esteem.
"You've got a 10-year-old who looks like a 14-year-old.
We interact with kids based on the way they look," Biro said.
"Kids interact with each other that way also."
"One of the things the parents of these early maturing kids need to do is they need to monitor them more closely," he said. That includes talking about sex earlier, but Juul said parents shouldn't be overly worried.
Age is decreasing
The data about early puberty and problems down the line come from studies that asked women when they first got their period, he said.
That typically happens two to four years after breasts start developing. It's unclear whether that age is decreasing at the same pace as the age of breast development, Juul said.
"I think the scary part is not (for) the actual girl, because we don't know what it means to her," he said.
"It's a warning that something is influencing our child population and it calls for concern and continued studies."
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