Children start out with high body fat and tend to get leaner as they age. Girls and boys' body compositions differ as well. To take age and sex differences into account, scientists have created a special BMI for children, called BMI-for-age.
Using a set of growth charts, doctors track the development of young people ages 2 to 20. The BMI-for-age figures in height, weight, and age to determine how much body fat a child has, comparing the results to those of others of the same age and sex. The calculation can help predict whether children will be at risk of being overweight when they're older.
For example, the normal BMI range increases for girls as they mature, because teenage girls normally have more body fat than do boys that age. A boy and girl of the same age might have the same BMI, but the girl's weight could be normal, while the boy could be at risk of being overweight.
Doctors stress that it's important to track a child's BMI over time rather than looking at one discrete number, because children can experience growth spurts.