"Now, for the first time, we're seeing a significant decrease in childhood obesity" nationally, said Thomas Frieden, CDC director.
Still, 1 in 8 preschoolers are obese in the United States, and it's even more common in black and Hispanic kids.
Preschoolers who are overweight or obese are five times more likely than other children to be heavy as adults, which means greater risks of high cholesterol, high blood sugar, asthma and even mental health problems.
Utah wasn't included in the study, which was based on height and weight data on nearly 12 million low-income children in 40 states. The children were ages 2, 3 and 4. Most were enrolled in the federal Women, Infants and Children (WIC) program which provides food vouchers and other services.
Utah's WIC data system was being upgraded during the study period, said Patrice Isabella, a nutrition coordinator at the Utah Department of Health.
Utah does have WIC data for 2009, which shows about 8.8 percent of preschoolers in the program were obese, compared with 12 percent nationally.
But data collected from a random sample of public schools suggests no change from 2008 to 2012 in the number of first and fifth graders in Utah who are overweight or obese, said Isabella.
It's harder to get national data on preschoolers of more affluent families, so it's not clear if the national trend applies to all young children. But experts note that low-income kids tend to be heavier.
"If you're going to look at the problem of obesity early in childhood, the group at highest risk are low-income kids. That's what makes this data so valuable for understanding trends in this major public health problem," said Dr. Matthew Davis, a University of Michigan researcher who tracks health policy and children's health issues.
The biggest declines were in Florida, Georgia, Missouri, New Jersey and South Dakota. Each saw their obesity numbers fall at least 1 percentage point.
Despite the improvements, the numbers are still disappointing. Hawaii was the best, with about 9 percent of low-income preschoolers estimated to be obese in 2011. California was worst, at nearly 17 percent.
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 had no significant change. Three 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 achieved success.
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.