ATLANTA — After several years in which the rate of obesity was determined to be on the upswing, new research from the Centers for Disease Control and Prevention suggests a shift in direction — at least for 2- to 4-year olds from low income families. It marks the first national data to show obesity and extreme obesity may be declining in young children, the C.D.C. said.

The prevalence of obesity among 2- to 4-year olds from low income families fell to 14.94% in 2010, down from 15.21% in 2003 and compared with 13.05% in 1998. Meanwhile, the prevalence of extreme obesity fell to 2.07% in 2010 from 2.22% in 2003 and compared with 1.75% in 1998, according to a researcher note written by the C.D.C. and published in the Journal of the American Medical Association.

“Obesity and extreme obesity in childhood, which are more prevalent among minority and low-income families, have been associated with other cardiovascular risk factors, increased health care costs, and premature death,” wrote Liping Pan, M.D., M.P.H., of the C.D.C., and colleagues. “Obesity and extreme obesity during early childhood are likely to continue into adulthood. Understanding trends in extreme obesity is important because the prevalence of cardiovascular risk factors increases with severity of childhood obesity.”

The research was based on data from the Pediatric Nutrition Surveillance System (PedNSS), which includes almost 50% of children eligible for federally funded maternal and child health and nutrition programs. The analysis for the study included 26.7 million children ages 2 through 4 years from 30 states and the District of Columbia that consistently reported data to PedNSS from 1998 through 2010. One routine clinic visit with demographic information and measured height and weight was randomly selected for each child.

The children were considered obese if their body mass index (B.M.I.), which is a measure of height and weight, was at or above the 95th percentile for children of the same age and gender. Children were considered to be extremely obese if their B.M.I. was 120% or above the 95th percentile.

The C.D.C. noted that the 2010 study population was slightly younger and had proportionally more Hispanics and fewer non-Hispanic whites and blacks compared with the 1998 population.

“To our knowledge, this is the first national study to show that the prevalence of obesity and extreme obesity among young U.S. children may have begun to decline,” the researchers said. “The results of this study indicate modest recent progress of obesity prevention among young children. These findings may have important health implications because of the lifelong health risks of obesity and extreme obesity in early childhood.”