We’ve all heard the news: We’re getting fat. Americans are inactive, McDonald’s-eating smokers with diabetes, right? That’s certainly a generalization, but you know what they say. Where there’s smoke, there’s fire. Tons of research dollars have been poured into studying this historic obesity epidemic. While some may imagine that obesity begins once a child is tall enough to reach the top shelf where mom and dad keep the cookies, a new study points to an even earlier age that jump starts obesity: infancy.

The first six months of a child’s life could determine whether or not he is obese by age three, the study says. Researchers from the Department of Ambulatory Care and Prevention at Harvard Medical School, Harvard Pilgrim Health Care, and Children’s Hospital Boston isolated a subset of 559 parent/infant pairs from the ongoing Project Viva, which studies women and their children. Previous studies had looked only at weight gain, but this report examined body length in addition to the pounds on the scale. They also analyzed growth as a dynamic process, measuring not only how much a child grows, but how quickly. All together, this data gave researchers a more accurate, balanced view of weight gain in the early years.

The results were clear: Take two babies with the same birth weight. Say that after six months, one weighs 16.9 pounds and the other, 18.4 pounds. That’s only a 1.5 pound difference. Check back a couple of years later and that child who weighed 18.4 pounds is 40% more likely to be obese at age 3.

“At first it may seem implausible that weight gain over just a few months early in infancy could have long-term health consequences, but it makes sense because so much of human development takes place during that period—and even before birth,” says Matthew Gillman, senior author on the paper. “Now we need to find out how to modify weight gain in infancy in ways that balance the needs of the brain and the body.”

A couple of caveats: though ethnically diverse, the study group was relatively homogeneous socioeconomically. A more comprehensive study would need to be done across socioeconomic lines to see if the results could be generalized. Secondly, it could be relevant to observe social and behavioral interactions around feeding between parents and infants.

In infancy, weight gain is not under a child’s control. Young babies can barely crawl, let alone reach the cookie jars. So mom and dad, listen up! While nobody wants to see WeightWatchers for Little Ones ads or waifish baby models on television, nobody wants to see a 200 pound two-year-old, either. Researchers urge early intervention and nutrition education. Have a healthy, growing baby, not a mini-sumo wrestler, and help curb the obesity epidemic.