WHAT IS BODY MASS INDEX (BMI)?
Simply put, BMI is a calculation of a person’s weight in relation to her height.
In order to find your child’s BMI percentile, a clinician calculates her BMI and plots that number on a chart for her age and gender. The clinician uses separate charts for boys and girls that take into account their natural differences in body fat. Your child’s resulting BMI percentile corresponds to one of the following categories:
- healthy weight
What do the BMI percentiles mean?
- underweight: below the 3rd percentile
- healthy weight: 3rd percentile to below 85th percentile
- overweight: 85th percentile to less than 95th percentile
- obese: 95th percentile and above
Some clinicians now consider a child with a BMI at or above the 99th percentile to be morbidly obese. However, the government does not currently recognize this category.
How does weight normally change relative to height in childhood?
This depends on a number of factors, including your child’s age and gender. The general guideline is that a child typically gains about three to five pounds with every inch she grows.
How do pediatricians screen for risk of obesity?
There are several tools your doctor might use to determine if your child is at risk for obesity, including:
- plotting your child’s BMI percentile yearly to see if there's a sudden increase
- identifying babies who gain weight too rapidly in infancy; studies show that these babies are at risk for becoming overweight as they get older
- birth weight and gestational diabetes: these factors may increase a child’s risk for obesity and type 2 diabetes later in life
- a family history of obesity, type 2 diabetes, hyperlipidemia, hypertension, sleep apnea and early heart attack: these factors can also put a child at increased risk for obesity and associated medical complications
How can I calculate my child’s BMI?
The easiest way to calculate your child’s BMI percentile is to use a BMI calculator on the Internet.
Why do we measure BMI differently in kids and adults?
We use BMI percentiles in children because they're still growing and developing, and the relationship among their height, weight and body fat changes as they grow. For example, girls going through puberty have more body fat than girls who haven't yet reached puberty. Generally speaking, the BMI percentile chart accounts for these kinds of changes.
Are there other ways to assess my child’s weight status?
- A triceps skinfold test is a measure of body fat. This test isn’t widely used in a clinical setting because results can vary depending on the person taking the measure, and research has shown it’s not a precise measure of body fat.
- A waist-to-hip ratio measures body fat distribution. Although the measure does correlate with risk factors for heart disease, it's not used routinely.
- A clinician may also ask about your child’s usual diet, your family history for being overweight and related medical problems, and her overall physical activity level to determine if your child is overweight or at risk for becoming overweight.
Why do some schools send home a BMI report card?
Many states, including Massachusetts, now require public schools to screen for BMI and alert parents to the result via a BMI report card. These reports cards are meant to let parents know if their child is clinically overweight and at risk for weight-related medical problems.
If your child's BMI report card tells you that your child is overweight or obese, discuss the results with your child’s pediatrician, who can help you to identify resources or programs, such as Boston Children’s Optimal Weight for Life (OWL) Program, which can help.
Are there any problems with using BMI to screen for obesity?
Yes. BMI doesn’t account for muscle mass, so a child with excess muscle mass might fall into an overweight or obese category, but it’s rare. Secondly, BMI doesn't provide any information about fat distribution, which correlates with risk for heart disease. A child with central adiposity (fat distributed primarily around the abdomen) has a greater risk for heart disease than a child with fat distributed primarily around the hips. A waist-to-hip ratio is used to measure body fat distribution.
What can I do if my child’s BMI is in the overweight or obese category?
There are many steps you can take as a parent to help your child achieve a healthy BMI:
- Be a positive role model by eating healthy foods and leading an active lifestyle.
- Only allow healthy food at home.
- Sit down to family meals.
- Serve age-appropriate portion sizes.
- Limit TV time to two hours or less a day.
- Encourage physical activity.
- Address any emotional factors that might be causing your child to overeat.