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CHICAGO - Childhood obesity in the United States appears to be causing girls to reach puberty at an earlier age, for reasons that are not clear, a study said on Monday.

An overweight pedestrian sits on a wall in a file photo. Childhood obesity in the United States appears to be causing girls to reach puberty at an earlier age, for reasons that are not clear, a study said on Monday. [Reuters]

The report from the University of Michigan's Mott Children's Hospital said a multiyear study following a group of 354 girls found that those who were fatter at age 3 and who gained weight during the next three years reached puberty, as defined by breast development, by age 9.

"Our finding that increased body fatness is associated with the earlier onset of puberty provides additional evidence that growing rates of obesity among children in this country may be contributing to the trend of early maturation in girls," said Dr. Joyce Lee, the lead author.

"Previous studies had found that girls who have earlier puberty tend to have higher body mass index, but it was unclear whether puberty led to the weight gain or weight gain led to the earlier onset of puberty," she added.

"Our study offers evidence that it is the latter," Lee said.

Earlier studies have found that US girls are reaching puberty earlier than was the case 30 years ago, a time span during which rates of childhood obesity also increased, the study said.

In the study girls were classified as at risk for being overweight if their body mass index (a measurement of weight related to age and height) was between the 85th and 95th percentiles, and defined as overweight if the measurement was greater than the 95th percentile.

The researchers said that 168 of the girls were classified as being "in puberty" by the age of 9 and nearly two dozen reported having their first menstrual period by two years later.

Higher body mass index scores at all ages had a "strong association with earlier onset of puberty, the authors said.

The study was published in Pediatrics, the journal of the American Academy of Pediatrics.

"Earlier onset of puberty in girls has been associated with a number of adverse outcomes, including psychiatric disorders and deficits in psychosocial functioning, earlier initiation of alcohol use, sexual intercourse and teenage pregnancy and increased rates of adult obesity and reproductive cancers," the study said.