What Parents Should Know About BMI

As children grow and their bodies change, it's not always easy for parents to tell if a child falls within a healthy weight range. Body mass index, or BMI, is a way of describing height and weight in one number that can help tell if someone's weight is healthy.

The CDC and the American Academy of Pediatrics recommend BMI screenings for all kids ages 2 and older. Here's what you need to know about checking on your child's BMI and what to do once you know it.

What Is BMI for Kids?

BMI estimates how much body fat you have. It's based on height and weight. But for kids, height and weight alone aren't as accurate as they are for adults. Why? Because kids' body fat percentages change as they grow. Their BMIs vary based on their age and gender.

That's why when health care professionals talk about a child's BMI, you won't usually hear a plain number, like 25, but rather a percentile, like 75th. They show how a child's BMI compares to other children of the same age and gender. To calculate the BMI percentile -- which is also called "BMI for age" -- a health care provider or an online tool like WebMD’s FIT Kids BMI Calculator takes a kid's BMI (along with age and gender) and looks it up on a pediatric growth curve. This gives the child's BMI percentile.

BMI percentiles are grouped into weight categories:

  • Underweight: below the 5th percentile
  • Healthy Weight: 5th percentile to the 85th percentile
  • Overweight: 85th percentile to the 95th percentile
  • Obese: 95th percentile or higher

For example, a 6-year-old boy with a 75th percentile BMI has a higher BMI than 75 out of 100 6-year-old boys. That’s in a healthy weight range.

Talking With Your Pediatrician About BMI

Many parents assume that if their child had a high BMI, their pediatrician would tell them. But that's not always the case. Sometimes doctors may not bring up weight issues with parents. So if you're interested in your child's BMI percentile, it's best to ask directly.

Some school districts have started to measure all children's BMIs in school. The school then sends home a report card to alert parents to any weight issues. Although some parents don't like the idea of schools sending reports of their child's BMI, experts say that the point is not to embarrass anyone. It's to let parents know about a health problem with serious consequences.

Studies from the U.K. show that children's BMI report cards can work. One study found that after getting the report, about 50% of the parents with overweight children made some healthy changes to their lifestyle.

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How Accurate is BMI for Kids?

Experts generally consider BMI for kids to be a good measure of body fat, at least among heavier children. But in some cases it might be misleading. Athletic kids, in particular, may fall into the overweight category when they are actually muscular.

Your child's BMI is important, but it is only a piece of the picture. If a BMI percentile indicates that your child is not within the healthy range, she needs a complete weight and lifestyle evaluation with a pediatrician.

Tips for a BMI Percentile in the Healthy Range

Experts recommend that kids of all ages and all weight categories follow these healthy guidelines to keep weight in check. It's easy to remember them as 5-2-1-0 every day.

  • 5: Everyone in your family needs five servings of vegetables and fruits. Keep serving them even if kids don't eat them. If they see a food over and over, they’re more likely to try it eventually. Give a fruit or vegetable with every snack or meal.
  • 2: Limit TV-watching to no more than 2 hours a day. Family members who use other "screens" -- video games or computers, for instance -- get less TV time. And kick the TV out of all bedrooms.
  • 1: Get 1 hour of physical activity. Add up the minutes each family member is moving -- it should be 60 minutes or more for each person. Start small and keep adding if necessary. The goal is to have all those minutes be at least moderate activity, sweating after about 10 minutes.
  • 0: That's how many sugar-sweetened beverages you should have a day. Juice drinks such as lemonade and fruit punch, sodas, tea, and coffee can all have added sugar. Stick to water and reduced-fat milk instead.
WebMD Medical Reference Reviewed by Roy Benaroch, MD on February 16, 2016

Sources

SOURCES:

Lawrence Cheskin, MD, associate professor, Johns Hopkins Medical School; director, Johns Hopkins Weight Management Center, Baltimore.

William H. Dietz, MD, PhD, director, division of nutrition, physical activity, and obesity, National Center for Chronic Disease Prevention and Health Promotion, CDC, Atlanta.

Karen Donato, SM, coordinator, overweight and obesity research applications, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD.

Dan Kirschenbaum, PhD, vice president, clinical services, Wellspring -- a Division of CRC Health; director, Center for Behavioral Medicine & Sport Psychology, Chicago; professor, psychiatry and behavioral sciences, Northwestern University Medical School, Chicago.

Ann O. Scheimann, MD, assistant professor of pediatrics, Johns Hopkins Children's Center, Baltimore.

CDC: "About BMI for Children and Teens."

CRC Health Group: My Overweight Child: "Knowing When to Intervene in the Life an Overweight Child."

Barlow, S. Pediatrics, 2007.

Healthy Children.org: "Is Your Child Overweight?"

Freedman, D. Pediatrics, September, 2009.

"Expert Committee Recommendations of the Assessment, Prevention and Treatment of Child and Adolescent Overweight & Obesity," 2007.

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