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Ten states were not included; some had changed how they track height and weight. One of the missing states is Texas, which has one of the largest populations of low-income children and is known to have a significant problem with childhood obesity.
Of the remaining 40 states, 18 showed at least slight improvement and 19 states and Puerto Rico had no significant change. Three states — Colorado, Pennsylvania and Tennessee — increased.
The last CDC study to look at childhood obesity data this way found very different results. From 2003 to 2008, significant declines in preschooler obesity were seen in only nine states and increases were seen in 24 states.
"We're seeing great progress," said the CDC's Ashleigh May, lead author of the new study.
The report didn't answer why some states improved while most others held steady, and Davis said there's a pressing need to do more research and understand how some states were able to scale back.
CDC officials said a change in WIC policies probably played a major role. The changes — instituted in 2009 — eliminated juice from infant food packages, provided less saturated fat, and made it easier to buy fruits and vegetables. Breast-feeding rates have been increasing, and kid's raised on mother's milk tend to have lower obesity rates, experts said.
Childhood obesity has been a focus of the Obama White House, with first lady Michelle Obama leading a campaign called "Let's Move!" Kass, the program's chief administrator, said thousands of preschools and day care centers across the country have pledged to increase physical playtime and serve healthier foods.
"While this announcement reflects important progress, we also know that there is tremendous work still to be done to support healthy futures for all our children," Michelle Obama said in a statement.