In the past, doctors used height and weight charts to track kids' growth, and compare it to other children the same age. But now, doctors use a measurement called body mass index, or BMI, to help estimate how much body fat children have. By tracking children's BMI over time on a growth chart, doctors can determine if their weight in relation to their height is appropriate for their age.
Calculation
To calculate kids' BMIs, doctors measure their weight and their height. Then, they plug these measurements into a standard formula to calculate the BMI. A child's BMI results are then compared with other children of the same age and gender. This is important, since the amount of body fat varies with a child's age and gender, so adult BMI calculators and ranges do not apply to children. Rather than using BMI ranges, doctor's plot a child's BMI on a standard growth chart to determine if the child's growth rate is healthy.
Significance
BMI helps doctors determine how much body fat a child has. There are more accurate ways of measuring body fat, such as underwater weighing or dual energy X-ray absorptiometry, but a BMI calculation is a quick, simple and inexpensive way to screen for weight problems in children. Generally beginning at age two, doctors calculate children's BMI to screen for weight problems in children, specifically overweight and obesity.
Percentiles
The chart that doctors use to track kids' BMIs determines their "percentile ranking" for their age and gender. Children whose percentile range is less than the 5th percentile are considered underweight. Those whose range is between the 5th percentile and the 85th percentile are considered to have a healthy weight. Children are considered overweight if their BMI percentile range is between the 85th and 95th percentile. If a child's percentile range is the 95th percentile or higher, she is considered to be obese.
Further Testing
It is important to remember that BMI is a screening calculation, and just because a child's BMI indicates that he has a weight problem doesn't mean that there is one. To accurately determine if children have a weight problem, doctors must perform further tests, which often include skinfold measurements, or assessments of diet, activity and family history.
Follow-up
Children's growth rates can vary significantly throughout childhood, so one high BMI measurement is usually not cause for concern. Doctors track BMI measurements over a period of several visits to determine if the BMI pattern indicates a child is at risk of becoming overweight or obese later on. For children who are considered at-risk of overweight or obesity, doctors carefully monitor their body fat, eating and exercise habits, and develop strategies to reduce their risk of weight problems later in life.
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