Regular checkups help the doctor monitor your child's growth and development. For years, doctors have used height and weight measurements as their primary tools for assessing a child's physical growth in relation to other children of the same age.
Now they have another tool: body mass index (BMI). Why is this new measurement so helpful?
What Is BMI?
BMI is a calculation that uses a child's height and weight to estimate how much body fat he or she has. Doctors use BMI to determine how appropriate a child's weight is for a certain height and age.
The best way to determine your child's BMI is to have your child's doctor do it. That way, you'll know the number is accurate and your child's doctor can discuss the result with you. If you're interested, you also can figure out your BMI by using this calculator:
Starting when your child is 2 years old, the doctor will probably determine his or her BMI at every routine checkup. The doctor will likely plot this measurement on a chart against those of other children who are the same age. Because what is normal changes with age, doctors must plot children's BMI measurements on standard growth charts rather than using a universal normal range for BMI as is done with adults. They also use separate charts for boys and girls to account for differences in growth rates and amounts of body fat as the two genders mature.
That information is recorded in your child's medical record, and over several visits, the pattern of measurements allows the doctor to track your child's growth.
BMI is particularly helpful for identifying children and adolescents who are at risk for becoming significantly overweight as they get older. In older children and teens, there is a strong correlation between BMI and the amount of body fat. Therefore, those with high BMI readings - and probably high levels of fat - are most likely to have weight problems when they are older. If doctors can identify these at-risk children early on, they can monitor their body fat more carefully and potentially prevent adult obesity through changes in eating and exercise habits.
What Do These Figures Mean?
A child's BMI percentile is a way of showing how his or her measurements compare to kids who are the same gender and age. For example, if a child has a BMI in the 60th percentile, 60% of kids of the same gender and age have a lower BMI.
BMI is not perfect. For example, it's very common for kids to gain weight quickly - and see the BMI go up - during puberty. Your child's doctor can help you figure out whether this weight gain is a normal part of development or whether it's something to be concerned about. If you think your child may be gaining or losing weight too fast, talk to your child's doctor. A child can also have a high BMI because he or she has a large frame or a lot of muscle, not excess fat. By the same token, a person with a small frame may have a normal BMI but might have too much body fat.
Although BMI is not a direct or perfect measure of body fat, a child above the 95th percentile is considered overweight because 95% of the population has a BMI less than he or she does. A child whose BMI is at the 50th percentile is close to the average of the population. A child below the 5th percentile is considered underweight because 95% of the population has a higher BMI.
Also, it's important to look at the BMI numbers as a trend instead of focusing on individual numbers. Any one measurement, taken out of context, might give you the wrong impression of your child's growth. The real value of BMI measurements lies in viewing them as a pattern over time. That allows both doctor and parents to watch the child's growth and determine whether it's normal compared with that of other children the same age. BMI is an important additional tool that can be used as an indicator that your child is growing and developing in a healthy way.
Reviewed by: Steven Dowshen, MD
Date reviewed: May 2005