Fat? Not my kid.

Most parents incorrectly identified their overweight preschool-age children as being at about the right weight, even as childhood obesity rates rose, according to alarming new research which tracked two sets of kids over separate multi-year periods.

While previous studies found parents commonly misperceived their children’s unhealthy weight as healthy, the latest study is described by its authors as the first to look at the lack of change in parents’ perception of preschoolers’ weight over time.

“There’s a declining tendency of parents to perceive an overweight or obese child as overweight,” says Dustin Duncan, an assistant professor of population health at New York University School of Medicine and the study’s lead author. “We need clear communication between pediatricians and parents for parents to understand their child’s weight status and implications for them to make healthful changes.”

Parents who fail to recognize their children are overweight or obese are less likely to take steps to help them maintain a healthy weight, Duncan says.

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“It shows that essentially we’re more obese as a society and we’re not recognizing our obesity as a society, in this case in children,” he added. “Obesity is a well-known medical condition associated with immediate and long-term health risks for children. This is an alarming finding.”

The nationally representative study, published last week online in Childhood Obesity, looked at two groups of more than 3,000 children ages 2 to 5: one from 1988-1994, and the other from 2007-2012.

Parents were asked if their children were overweight, underweight or just about the right weight. The children in the later group were significantly more overweight than those in the first but parents’ perceptions of their kids’ weight were roughly the same.

Some 97 percent of parents of overweight boys in the earlier study group identified their sons as about the right weight, compared to 95 percent in the second group. With the overweight girls, 88 percent of parents in the first group thought they were just about right, compared to 93 percent in the later group.

The study found children in the more recent study group were 30 percent less likely to be correctly perceived as overweight, Duncan says.

“In addition to finding that parents incorrectly misperceived their overweight children as the right weight in the older, earlier survey, that persisted over time, despite childhood obesity increasing over the study period,” Duncan says.

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A potential explanation, he says, is that the social comparison theory is at play, meaning parents are comparing their child to children around them, rather than consulting the child’s doctor and growth chart.

“People are judging their child based on friends and neighbors, who are likely to also be overweight and obese,” Duncan says.

NBC medical contributor Dr. Natalie Azar discussed the study on TODAY Monday, saying the preschool years are when healthy habits can take hold.

“Parents have this idea that children are going to outgrow obesity and I think that’s why they are more reluctant to acknowledge it,” she told Savannah Guthrie. “We know really importantly that these habits that children learn start very young. In the preschool age is when these healthy behaviors are going to form.”

Azar also stressed the need for parents to consult their pediatricians regarding weight.

“It’s communication with pediatrician that’s so important,” she said. “It’s not so much an indictment on the part of the medical community, but clearly there’s been a little bit of a lack of communication between the doctors and the parents together to form a uniform task force.

“Because parents need to feel empowered, and they have to say, ‘Listen, I can’t ignore the facts any more. It appears that my child is in this range,’” she said. “We have to start intervening early.”

Lisa A. Flam, a regular contributor to TODAY.com, is a news and lifestyles reporter in New York. Follow her on Twitter.