Chubby babies and toddlers at risk for later obesity are on the decline in a government food program serving millions of kids, a glimmer of good news in the nation’s fight to slim down. The trend was found in a study on children up to age 3 enrolled in the WIC nutrition assistance program for low-income women and children. Half of all U.S. infants up to 12 months old are enrolled in the program. The portion of youngsters at risk for obesity fell during the study, from almost 15 percent in 2010 to 12 percent overall in 2014, researchers from the federal Centers for Disease Control and Prevention reported. The rate declined in all ages studied. It was lowest — about eight percent in 2014 — for the youngest infants, aged 3- to 5 months, versus almost 15 percent among toddlers. The results echo a reported decline in older WIC children and were unexpected, given rising rates earlier in the decade, said CDC researcher David Freedman, the lead author. “People are thrilled,” he said. Freedman said reasons are uncertain for the decline in heavy WIC babies, but it came amid changes designed to improve nutrition and health in WIC food packages, including more whole grains, fruits and vegetables. Also, breastfeeding among participating women increased in 2009 and that can protect against obesity. The study was published Tuesday in Pediatrics. Doctors don’t usually describe babies as obese, but measure their risk using a weight-for-length ratio. Those with a high ratio, generally heavier than 95 percent of their peers, face an increased chance of becoming obese later on. Whether the decline has continued is uncertain and the study didn’t track infants to see if they became overweight or obese later on. According to the CDC, U.S. obesity rates total almost 18 percent among all 6- to 11-year-olds and nearly 21 percent among 12- to 19-year-olds.