However, the very young children constitute a tiny fraction of the US population and the figures for the broader society have remained flat, and even increased for women over 60.
Obesity among 2- to 5-year-olds dropped to 8 per cent in 2011-2012 from 14 per cent in 2003-2004, a major federal health survey found.
"This is the first time we've seen any indication of any significant decrease in any group," said Cynthia L Ogden, a researcher for the Centres for Disease Control and Prevention, and the lead author of the report.
Childhood obesity has been shown to increase risk of obesity, cancer, heart disease and stroke later in life, the report said.
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The study weighed and measured an estimated nearly 9,000 people, including about 600 infants and toddlers.
A third of US adults and 17 per cent of youths are obese, the survey found.
The study was published in the Journal of the American Medical Association (JAMA).