No age group -- not even infants under 6 months of age -- has escaped the alarming trend, according to Harvard Medical School Associate Professor Matthew Gillman, senior author of the study, which is published in the journal Obesity.

The researchers analyzed height and weight measurements from medical records of more than 120,000 children younger than six years old spanning 1980 to 2001.

The prevalence of being overweight among infants younger than 6 months increased from 3.4 percent to 5.9 percent during the 22-year period -- an increase of more than 73 percent.

This finding is particularly troubling because faster weight gain in the first few months after birth has been linked to obesity later in life.

In an effort to explain the trend, the researchers cited several possible factors: More mothers now are overweight when they become pregnant, compared to a quarter-century ago, and developing gestational diabetes.

This can result in babies born large for their gestational age. Also, more babies now are experiencing rapid weight gain during their first few months of life.

The study shows that obesity prevention must start before birth, Gillman said. Specifically, pregnant women should not smoke, and they should take steps to avoid excessive weight gain and gestational diabetes.

He also encouraged women to breastfeed. Not only is breastfeeding generally accepted as the optimal nutrition for infants -- it also helps new mothers shed some of the weight gained during pregnancy.

The researchers used the Centers for Disease Control and Prevention's measure for a child being overweight: above the 95th percentile for age and gender on a weight-for-height index.