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 he is at risk for health problems based on his weight. Measuring the waist or neck circumference is another way to measure body fat in kids. Your health care provider 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. Kids' 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 BMI number, like 25, but rather a BMI percentile, like 75th. These BMI percentiles 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 up to the 85th percentile
- Overweight: 85th percentile up to the 95th percentile
- Obese: 95th percentile or higher
So, 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. And though you may think that means he is heavy, he is considered a healthy weight.
Talking With Your Pediatrician About BMI for Kids
Many parents assume that if their child had a high BMI, their pediatrician would tell them. But 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 school. The school then sends home a BMI report card to alert parents to any weight issues. Although some parents don't 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 study found that after getting a BMI report, about 50% of the parents with overweight children made some healthy changes to their lifestyle.