Doctors use BMI to determine whether a child is at an appropriate weight for that individual's height, states Kids Health. BMI, which stands for body mass index, estimates how much body fat a person has based on height and weight.
BMI is assessed for children, teens and adults using the same formula and regardless of gender, according to the CDC. Doctors may use a BMI-for-age percentile growth chart to track where a child stands in relation to other children and teenagers in the United States. It is not a diagnostic tool. Instead, BMI is used to screen for potential weight-related health problems. To determine whether a high BMI number results from body fat or muscle mass, a doctor needs to measure the thickness of the skin folds, evaluate diet, examine family history and perform other tests.
Although the formula for BMI is the same across age groups, the CDC states that because children are growing, their BMI is calculated in relation to each other instead of to a standard. BMI is unable to determine what a healthy weight is because of all the different factors such as weight, height, age and gender; the chart loosely states that children should be between the fifth and 85th percentile of their age groups.