The finding comes from a government study considered a gold-standard gauge of trends in the public's health. The researchers found that obesity among children ages 2 to 5 decreased - to 8 percent, from 14 percent a decade ago. That would represent a 43 percent drop.
But the only decline was seen in preschoolers, not in older children. And some experts note that even the improvement in toddlers wasn't a steady decline, and say it's hard to know yet whether preschooler weight figures are permanently curving down or merely jumping around.
It is enough of a decline to be optimistic, said Cynthia Ogden, one of the study's authors.
"There's a glimmer of hope," said Ogden, an epidemiologist with the Centers for Disease Control and Prevention.
The report was published online Tuesday in the Journal of the American Medical Association.
Health officials have long been hoping for more substantial evidence that they've turned a corner in the fight against childhood obesity.
Obesity is seen as one of the nation's leading public health problems - health officials call it a longstanding epidemic. A third of U.S. children and teens and more than two-thirds of adults are obese or overweight.
Officials are particularly worried about the problem in young children. 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.
After decades on the rise, childhood obesity rates recently have been flat. But a few places - including New York City and Mississippi - reported improvements in the last couple of years. Seattle joined that list last week, with a report of recently declining obesity in older school children in low-income school districts.
More broadly, health officials last year reported at least slight drops in obesity for low-income preschoolers in 18 states. But they mainly were children enrolled in the federal Women, Infants and Children (WIC) program, which provides food vouchers and other services. Experts attributed the improvement to WIC policy changes in 2009 that eliminated juice from infant food packages, provided less saturated fat, and made it easier to buy fruits and vegetables.
The new study is a national survey of about 9,100 people - including nearly 600 infants and toddlers - in 2011-2012, in which participants were not only interviewed but weighed and measured. The results were compared to four similar surveys that stretched back to 2003.
"I think it's fair to say that (this study) is probably the best source of data we have on whether the prevalence of obesity is increasing with time," said Dr. Robert C. Whitaker, a Temple University expert of childhood obesity.
The main finding was that, overall, both adult and childhood obesity rates have held flat in the past decade. And there were no significant changes in most age groups.
But there were two exceptions: For some reason experts aren't sure about, the obesity rate in women age 60 and older rose from 31.5 percent to more than 38 percent. And the preschool obesity rate dropped.
Some health leaders in Washington, D.C., and Atlanta celebrated the latter finding. They say it's an early sign of a pay-off from campaigns to increase breastfeeding rates and cut consumption of sodas and other sugary beverages. First lady Michelle Obama issued a statement that her 'Let's Move!' initiative - which promotes youth exercise and good nutrition - is causing healthier habits "to become the new norm."
Some experts were more cautious about the results.
The preschooler obesity numbers fell from 14 percent in 2003-2004 to 10 percent in 2007-2008, then jumped to 12 in 2009-2010, then slipped to 8 in the most recent survey.
So it seems to have been jumping around a little. "We're going to need more" years of data to see if the apparent trend is really nosing downward, said John Jakicic, director of the University of Pittsburgh's Physical Activity and Weight Management Research Center.
Some wondered whether it makes sense that preschoolers would be the ones leading a downward trend in childhood obesity. For years, most childhood anti-obesity initiatives were older-kid efforts removing soda vending machines from schools and increasing physical education.
Apart from the WIC policy change, there's been less of a push regarding preschoolers. "Relative to older children, less has been done" to fight obesity in toddlers, Whitaker said.
Lingering questions aside, Jakicic said he was still glad to see the numbers. "I think we should be excited it's not getting worse," he said.