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Assessing Your Child's Weight with the Body Mass Index (BMI)

Regular checkups help the doctor monitor your child's growth and development. For years, doctors have used height and weight measurements as their primary tools for assessing a child's physical growth in relation to other children of the same age. Now they have the Body Mass Index (BMI), another tool to assess growth and to possibly indicate whether a child's body fat is at an unhealthy level. Read on to learn how the BMI fits into regular visits to your child's doctor.

What Is the Body Mass Index?

BMI is a measurement derived from a child's (or adult's) weight and height that helps the doctor determine whether the child's weight is appropriate for height.

Check out this easy calculator to determine your child's BMI.

BMI was only recently recommended as an additional measurement of growth. Doctors have been using the weight for stature (height) charts to assess the appropriateness of a child's weight, but that measurement is of limited value because it can only be used in boys younger than 11.5 and girls younger than 10 years.

Doctors have found BMI to be a more accurate reflection of body "fatness" and potential weight problems than comparing height and weight measurements. In addition, a doctor can chart a child's BMI from ages two through 20. Separate charts are used for boys and girls. Although some doctors do not use it yet, as charts for BMI become more widespread, you may soon see it charted during visits to your child's doctor, if you do not already.

BMI is particularly helpful for identifying children and adolescents who are at risk for becoming overweight as they get older. In older children and teens, there is a strong correlation between BMI and the amount of body fat. Therefore, those with high BMI readings–and probably high levels of fat–are most likely to have weight problems when they are older. If doctors can identify these at-risk children early on, they can monitor their body fat more carefully and potentially prevent adult obesity through changes in eating and exercise habits.

At Your Child's Doctor's Office
Starting when your child is 2 years old, the doctor will probably determine the BMI at every routine checkup. He or she will plot this measurement on a chart against those of other children who are the same age. Because what is normal changes with age (babies have more "baby fat," for instance), doctors must plot children's BMI measurements on standard charts for children rather than using a universal normal range for BMI as is done with all adults. They also use separate charts for boys and girls to account for differences in growth rates and amounts of body fat as the two genders mature. That information is recorded in your child's medical record, and over several visits the pattern of measurements allows the doctor to track your child's growth.

What Do These Figures Mean?

Although BMI is not a direct or perfect measure of body fat,  children with a BMI greater than or equal to the 85th percentile are considered overweight. Children with a BMI greater than or equal to the 95th percentile are considered obese.

It is important to look at BMI readings as a trend instead of focusing on individual numbers. Any one measurement, taken out of context, might give you the wrong impression of your child's growth or weight pattern. The real value of BMI measurements lies in viewing the pattern over time to determine whether the pattern is normal compared with other children the same age. BMI is an important additional tool that can be used as an indicator that your child is growing and developing in a healthy way.

See your doctor if you think your child has a serious weight problem.

Wondering about your own BMI? Calculate it here!

Reviewed by the Scientific Advisory Panel, 2006