Body mass index -- or BMI -- is a measure of the amount of fat on a person's body. According to the Centers for Disease Control and Prevention, BMI is calculated by dividing your child's weight in pounds by his height in inches squared, then multiplying that number by a conversion factor of 703. The result is a number that, along with guidance from your pediatrician, can be used to ensure healthy growth.
Purpose
BMI is measured using weight and height measurements. It does not measure fat directly, but a high BMI can correlate to the amount of fat on your child's body. BMI numbers are used to screen weight problems in children, typically beginning at 2 years of age, or look for indicators of a weight problem later in life.
Charts
At a doctor's visit, your child's height and weight is typically plotted on growth charts developed by the Centers for Disease Control and Prevention in order to keep track of size and growth patterns in the United States. These charts can help to indicate your child's BMI and are specific to age and gender.
Indicators
Your child is considered underweight if her BMI falls below the 5th percentile, according to the CDC. A healthy weight is anywhere between the 5th and 85th percentile. If her BMI is above the 85th percentile but less than the 95th percentile, she is considered overweight. A BMI equal to or greater than the 95th percentile is considered obese.
Factors
Your child's BMI is not necessarily a definite indicator of his health or body fat. If your child has a growth spurt and suddenly shoots up in height, he may have a lower BMI than others his age and gender. If he goes through puberty and gains some weight but not much height, his BMI may suddenly rise. It is also common for a child with a large frame or a lot of muscle to have a high BMI. Your child's doctor can help determine whether his BMI is in a healthy range.
Considerations
Because BMI is not the only indicator of body fat, your child's doctor may perform other tests to determine whether there is a weight problem. The doctor may measure your child's skin folds and evaluate her diet, physical activity and family history.
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