Boston ? Chubby cheeks and dimpled thighs have long been a mother’s proof of a healthy, well-fed baby. But those roly-poly infants now may be a sign of something much different: America’s problem with weight.

Even babies younger than 6 months are more likely to be overweight today compared with those 20 years ago, a study of Massachusetts children found.

Fewer than 1 in 10 babies was found to be too fat, but the rate was still up substantially from two decades ago. That’s worrisome because research has shown that accelerated weight gain in a child’s early months can predict weight problems and higher blood pressure later in life, said senior author Dr. Matthew Gillman, associate professor at Harvard Medical School.

“The results of this study point out very clearly that the origins of overweight are at the origins of human life, even at birth,” he said.

The researchers looked at medical records of more than 120,000 children who visited doctors from 1980 to 2001. All were enrolled in a health maintenance organization that used an electronic medical record system and most came from middle-class families.

Among children 6 months and younger – a group seldom included in weight studies – the percentage of overweight babies jumped from a little more than 3 percent in 1980 to nearly 6 percent.

Whether a baby was overweight was determined mostly by change in weight over those first crucial months – especially weight gain out of proportion with length – rather than the infant’s weight at a specific age, Gillman said.

For the group overall, the prevalence of overweight children increased from 6 percent to 10 percent.

Researchers didn’t study why there is a higher rate of fat babies today, but Gillman pointed to previous studies that have connected higher birth weights to mothers who were overweight before or during pregnancy and to those who had diabetes during pregnancy.

“Good habits need to begin at the very beginning of life,” said Dr. Louis Aronne, director of the Obesity Center at New York-Presbyterian Hospital/Weill Cornell Medical Center, who was not involved with the study.

Laura Riley, a doctor of obstetrics and gynecology at Massachusetts General Hospital in Boston who was not connected with the research, said she hoped it would give doctors more ammunition in the push to get mothers to make healthier choices before, during and immediately after pregnancy.