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 for short, is a standard measurement of body fat. Your child's BMI can help you determine if your child is at risk for health problems based on his weight. Measuring the neck circumference is another way to measure body fat in kids. Your doctor may also do this.
The CDC and the American Academy of Pediatrics recommend BMI screenings for all kids age 2 and older. Here's what you need to know about checking on your child's BMI and what to do with the info once you have it.
What Is BMI for Kids?
BMI estimates how much body fat you have. Calculating a child's BMI number starts out just like calculating an adult's BMI. 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 and depend on their age and gender.
That's why when you hear about a child's BMI, you don't usually get a plain BMI number, like 25, but instead you get a BMI percentile, like 75th percentile. These BMI percentiles show how a child's weight and height compare to other children of the same age and gender. For example, a boy with a 75th percentile BMI is heavier than 75 out of 100 boys his age. And though you may think that means the child is heavy, that child still falls within the healthy weight category. A healthy weight range is between the 5th and 84th percentiles.
To calculate the BMI percentile -- which is also called BMI-for-age -- a doctor or an online tool takes your child's BMI (based on height, weight) and plots it on a pediatric growth chart. This will give you your child's percentile. Each percentile falls into a specific weight category: underweight, healthy weight, overweight, or obese.
Here's how kids' BMI percentiles break down:
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Underweight: less than the 5th percentile
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Healthy Weight: 5th percentile up to the 85th percentile
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Overweight: 85th percentile to less than the 95th percentile
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Obese: 95th percentile or greater
Talking With Your Pediatrician About BMI for Kids
Many parents assume that if their child had a high BMI, their pediatrician would tell them. That's not necessarily the case. Sometimes pediatricians 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 each school. The school then sends home a BMI report card to alert parents to any weight issues.
Although not all parents like the idea of schools sending report cards with 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 found that after getting a BMI report, about 50% of the parents with overweight children made some healthy changes to their lifestyle.