CHICAGO (AP) — Researchers say there's a new way to tell if infants are likely to become obese later on: Check to see if they've passed two key milestones on doctors' growth charts by age 2.

Babies who grew that quickly face double the risk of being obese at age 5, compared with peers who grew more slowly, their study found. Rapid growers were also more likely to be obese at age 10, and infants whose chart numbers climbed that much during their first 6 months faced the greatest risks.

That kind of rapid growth should be a red flag to doctors, and a sign to parents that babies might be overfed or spending too much time in strollers and not enough crawling around, said pediatrician Dr. Elsie Taveras, the study's lead author and an obesity researcher at Harvard Medical School.

Contrary to the idea that chubby babies are the picture of health, the study bolsters evidence that "bigger is not better" in infants, she said.

But skeptics say not so fast. Babies often grow in spurts and flagging the speediest growers could lead to putting infants on diets — a bad idea that could backfire in the long run, said Dr. Michelle Lampl, director of Emory University's Center for the Study of Human Health.

"It reads like a very handy rule and sounds like it would be very useful — and that's my concern," Lampl said. The guide would be easy to use to justify feeding infants less and to unfairly label them as fat. It could also prompt feeding patterns that could lead to obesity later, she said.

Lampl noted that many infants studied crossed at least two key points on growth charts; yet only 12 percent were obese at age 5 and slightly more at age 10. Nationally, about 10 percent of preschool-aged children are obese, versus about 19 percent of those aged 6 to 11.

Lampl and Edward Frongillo, an infant growth specialist at the University of South Carolina, voiced concern in an editorial accompanying the study in the journal Archives of Pediatrics & Adolescent Medicine, released online Monday. They argue that more research is needed to confirm whether the study's recommendation is really a useful way to flag infants for obesity.

"The potential to do more harm than good is actually very high," Frongillo said.

Taveras said the kind of rapid growth noted in the study should be used to raise awareness about potential risks but is not a reason to put babies on a diet.

The study involved 45,000 infants and children younger than age 11 who had routine growth measurements during doctor checkups in the Boston area from 1980 through 2008.

Growth charts help pediatricians plot weight, length in babies and height in older kids in relation to other children their same age and sex. Pediatricians sometimes combine an infant's measures to calculate weight-for-length — the equivalent of body-mass index, or BMI, a height-to-weight ratio used in older children and adults.

The charts are organized into percentiles. For example, infants at the 75th percentile for weight are heavier than 75 percent of their peers.

The study authors used seven major cutoffs on the charts — the 5th, 10th, 25th, 50th, 75th, 90th and 95th percentiles — to calculate growth pace. An infant whose weight-for-length jumped from the 19th percentile at 1 month to the 77th at 6 months crossed three major percentiles — the 25th, 50th and 75th — and would be at risk for obesity later in childhood, the authors said.

Larger infants were most at risk for obesity later on, but even smaller babies whose growth crossed at least two percentiles were at greater risk than those who grew more slowly.

About 40 percent of infants crossed at least two percentiles by age 6 months. An analysis of more than one-third of the study children found that 64 percent grew that rapidly by age 2.

Dr. Joanna Lewis, a pediatrician at Advocate Lutheran General Hospital in Park Ridge, Ill., said she supports the idea that infancy is not too young to start thinking about obesity.

Still, she emphasized that rapid growth in infancy doesn't mean babies are doomed to become obese. "It's not a life sentence," and there are steps parents can take to keep their babies at a healthy weight without restrictive diets, she said.

Lewis said many of her patients are large babies whose parents feed them juice or solid food despite guidelines recommending nothing but breast milk or formula in the first six months.

"The study reinforces what we try to tell parents already: Delay starting solids and don't put juice in a bottle," Lewis said.

Lewis also advises parents that when starting infants on solid food, have the whole family sit down and eat together. Research has shown that obesity is less common in children raised in families that have frequent meals together at home.