BMI, or body mass index, is a single number used to assess a person's weight in relation to height. It is used to decide if someone's weight is appropriate for his or her height. In the past, BMI was used only for adults. Now, the U.S. Department of Health and Human Services has released new and updated growth charts for those from 2 to 20 years old. BMI will be used as a new assessment tool for children and teens. The most important change in the charts is the addition of BMI-for-age.

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About 25% of children and teens, ages 6 to 17, are overweight or obese. Childhood obesity increases the risk of adult obesity. Children who are obese at age 6 have a 50% or greater chance of becoming obese adults. The new charts that include BMI can help identify children who run the risk of becoming overweight or obese down the road. BMI serves as an early warning signal.

Most people are familiar with the growth charts developed in 1977 by the National Center for Health Statistics (NCHS). NCHS is now a part of the Centers for Disease Control and Prevention (CDC). The growth charts are a series of curves that show the ranges of growth for children in the United States. These charts are used to plot children's height and weight to make sure they are on the path to normal growth. The new charts are updated versions of these charts. Adding BMI makes the growth charts a much more useful and accurate tool.

The English formula for body mass index is:

BMI = weight in pounds divided by height in inches, divided by height in inches, multiplied by 703.

Example: A 35-pound child is 36 inches tall. Thirty-five pounds divided by 36 inches, divided by 36 inches, multiplied by 703 = 18.9 BMI.

The metric formula for body mass index is:

BMI = weight in kilograms (kg) divided by height in centimeters (cm), divided by height in centimeters, multiplied by 10,000.

Example: A 17-kg child is 106 cm tall. Seventeen kg divided by 106 cm, divided by 106 cm, multiplied by 10,000 = 15.1 BMI.

Once the BMI is calculated, this number and the child's age are used to plot a percentile on the BMI-for-age growth chart. There are separate charts for girls and boys because they differ in amount of body fat as they mature. The percentile determines whether the child is within a normal weight range, overweight, at risk of becoming overweight, or underweight. Growth charts can be found at the Web site for the Centers for Disease Control and Prevention, at www.cdc.gov/growthcharts. Information about BMI can be found at www.cdc.gov/nccdphp/dnpa/bmi/bmi-for-age.htm

BMI-for-age percentiles that cause some concern in those ages 2 to 20 are:

Besides being a tool for determining appropriate weight, BMI can help determine health risk. Sixty percent of children and teens with a BMI-for-age above the 95th percentile have at least one risk factor for cardiovascular disease. Twenty percent have two or more risk factors for the disease.

When used properly, these charts can explain patterns of growth and identify goals for positive change. They can be used to evaluate and support changes in weight and growth over time. What's important is the general growth pattern over time, not a single measurement plotted at any one visit. Growth charts using BMI are a great assessment tool. But they should not be the only means of evaluating a child's development.

Using BMI as part of growth charts is a good way to find a child's risk for becoming obese or overweight. This and other growth problems can be detected early. That means steps can be taken to better prevent excess weight gain in children. A healthcare provider can measure children to see where they stand on the new growth and BMI charts.