Young children naturally start out with high body fat, but tend to get leaner as they get older. Girls and boys also have different body compositions. To take into account the differences between boys and girls and children of different ages, scientists have created a special BMI for children, called BMI-for-age.
Doctors use a set of growth charts to track the development of children and young adults between the ages of two and 20. The BMI-for-age figures in a child's height, weight, and age to determine how much body fat he has. It compares the results to those of other children of the same age and gender, and can help predict whether children will be at risk for being overweight when they get older. You can view a complete set of the charts at CDC's growth chart page.
Each chart contains a set of curved lines indicating the child's percentile. For example, if a 15-year-old boy is in the 75th percentile for BMI, 75 percent of boys of the same age have a lower BMI. He is at a normal weight. Although his BMI changes as he grows, he can stay at around the same percentile and remain at a normal weight.
The normal BMI range becomes higher for girls as they mature, because teenage girls normally have more body fat than teenage boys. A boy and girl of the same age may have the same BMI, but the girl could be of normal weight and the boy could be at risk for being overweight.
Doctors say it's important with children to track BMI over time rather than looking at one individual number, because children can go through growth spurts.
In the next section, we'll learn about some of the controversy associated with using BMI.