WASHINGTON - Pediatricians are supposed to track if youngsters are putting on too many pounds - but a new study found less than a quarter of parents of overweight children recall the doctor ever saying there was a problem.

Does that mean doctors aren't screening enough kids, or aren't frank enough in these tough conversations? Or is the real story parent denial? The research published Monday can't tell, but makes it clear the message too often isn't getting through.

"It's tricky to say, and it's tricky to hear," says lead researcher Dr. Eliana Perrin of the University of North Carolina. She analyzed government health surveys that included nearly 5,000 parents of overweight children from 1999 to 2008.

Parents tend not to realize when a weight problem is creeping up on their children. When almost a third of U.S. children are at least overweight, and about 17 percent are obese, it's harder to notice that there's anything unusual about their own families. Plus, children change as they grow older.

The new study suggests when parents do recall a doctor noting the problem, it's been going on for a while.

About 30 percent of the parents of overweight 12- to 15-year-olds said a doctor had alerted them, compared with just 12 percent of the parents of overweight preschoolers. Even among the parents of very obese children, only 58 percent recalled a doctor discussing it, says the report published Monday by the journal Archives of Pediatrics & Adolescent Medicine.

"Many pediatricians don't worry until children are very overweight, or until they're much older," says Perrin, whose team has created stoplight-colored growth charts to help doctors explain when a problem's brewing. "If we can notice a concerning trend early, we're more likely to be able to do something about it."

That means taking a family approach, says Dr. Nazrat Mirza, medical director of an obesity clinic at Children's National Medical Center in Washington. Important changes - such as switching to low-fat milk and water instead of sugary sodas and juice, or cutting back on fast food - should be viewed as making the whole family healthier, not depriving everyone because Johnny needs to lose weight.

"You do not want to single out one individual in the family. That's enough to cause a lot of friction," says Mirza, who wasn't involved with the new study.

Doctors have long tracked children's height and weight during yearly checkups, but more recent guidelines urge them to calculate a youngster's body mass index, or BMI, to screen for developing obesity. Unlike with adults, one measurement alone doesn't necessarily mean children are overweight - they might be about to shoot up an inch.

The next step is plotting that BMI on a growth chart. Youngsters are considered overweight if their BMIs track in the 85th to 95th percentile for children their same age and gender, a range that just a few years ago was termed merely "at risk." Above the 95th percentile is considered obese.

To tackle lack of awareness, Children's National has begun calculating BMIs for every child age 2 or older who is admitted for any reason. Mirza calls it "a teachable moment."

Perrin's analysis shows more parents of overweight kids are starting to get the message. Overall, 22 percent of parents reported a health professional telling them their child was overweight. But that rose to 29 percent in 2008, the latest year of the survey data and about the time guidelines changed.

So what should parents, and overweight children themselves, be told?

Perrin focuses on health, not fat. She tells them the child is at an unhealthy weight that puts them at risk for later problems - and that she can help families learn to eat better and get more active. That's where her color-coded BMI charts (www.eatsmartmovemorenc.com) come in. Parents can tell at a glance if their child is in the overweight yellow zone or the obese red zone, and over time if they're moving closer to the green zone. Perrin calls the charts especially useful between ages 3 and 8, when children are growing so fast it's particularly hard to tell if they're a healthy size.

Portion size is key, too. Nutritionists define the right size as about 1 tablespoon of each food type for every year of age. Perrin's easier measure is that a serving is about the size of a child's palm, which will grow as the child gets older.

Pre-teens and teens are more independent and have to be on board, adds Mirza. Teens, for example, start to stay up late, eating more at night and skipping breakfast, not a healthy pattern. The kid who never exercises will tune out all weight advice if told to hit the gym but might agree to walk around the block. The athlete might be sabotaging physical activity with 600-calorie snacks.

The good news: As kids grow older and taller, "they can grow into a healthier weight," Perrin says. And "we know that parents with an accurate assessment of their child's weight are more likely to make weight-related changes."

Lauran Neergaard covers health and medical issues for The Associated Press in Washington.