Federal health authorites have decided to throw out the most recent weight data in figuring out whether a child is overweight

From CNN Medical Correspondent Elizabeth Cohen

(CNN) -- Government height and weight charts for children are getting a makeover. But in the course of updating the charts, doctors have run into a problem: Today's children are too fat.

Pediatricians use the charts to compare children the same age to one another.

If they use the new data, a heavy child may look fine on paper only because the child's peers are also fat.

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"This would basically tell us we had a number of youngsters who are not overweight who really are," said Dr. Robert Malina, an expert on children and physical activity at Michigan State University.

So federal health authorities have decided that as they re-configure the charts, they're going to throw out the most recent weight data.

The government has decided to use only weight data gathered before 1988, the date children age 6 and older started to become markedly heavier.

Before 1988, about 6 percent of less of American children ages 6 through 17 were overweight.

Since then, the number has jumped to about 11 percent of all children.

So why is it that so many more children are overweight?

The answer is painfully obvious, many children eat too much and exercise to little.

Doctors say that while it would be great if children participated in organized sports, it doesn't take that much to make a difference.

"Say, for example, a youngster would walk to school 10 or 15 minutes a day each way; that's 30 minutes of activity," Maline said. "Now youngsters are driven to school; that's 30 minutes of inactivity."

When they get home -- a double whammy. Many children are couch potatoes, watching ads for high fat foods on television.

The new growth charts are due out this fall. In some ways, they will be more up to date, they'll include more information on minority children, for example.

But when it comes to weight, doctors say it's healthier to compare today's children to yesterday's children.