An Explanation Of Bmi In A Child

Children experience growth spurts and alternating periods of rapid or stable growth throughout childhood. Boys and girls grow at different rates and even within the same sex, healthy children may grow at different rates. Their proportion of lean muscle mass and fat tissue changes naturally as children grow. The body mass index helps determine whether the proportion of body fat and muscle indicates a need for further nutritional and health assessment with other measures.

History

The use of BMI for assessing growth in children began in 2000 with new BMI-for- age-and-sex charts developed by the Centers for Disease Control and Prevention. Until BMI could be associated with age and sex, it did not provide useful information because a healthy BMI in children varies according to their growth pattern and differs between boys and girls. This problem was resolved with the new growth charts and, as of 2011, BMI for children is interpreted with respect to their growth curve rather than using the underweight, normal, overweight and obese BMI cutoff values used to categorize adults.

Identification

Just as for adults, your child's BMI is calculated as a ratio of her weight in kilograms divided by her height in meters, squared. BMI is easy to calculate and serves as a fairly reliable indirect measure of body fatness. Measuring and tracking BMI provides an inexpensive method for screening potential weight-related health problems, according to the CDC.

Calculation

BMI equals weight in kilograms divided by height in meters, squared. If your child was measured with a standard tape measure and bathroom scales, you may need to convert to metric units to use the standard formula. Divide his weight in pounds by 2.2 to convert to weight in kilograms. Multiply his height in inches by 2.54 to get height in centimeters, then divide that number by 100 to convert it to height in meters. Multiply the number of meters by itself to get meters squared.

If you are not comfortable with using metric units in the calculation, you can get similar results by dividing your child's weight in pounds by her height in inches, squared, and multiplying the result by the constant, 705.

Interpretation

Plot your child's BMI on a Body Mass Index-for-Age Percentiles CDC growth chart for boys or for girls, as appropriate for your child. BMI is interpreted differently for children than for adults because BMI is age- and sex-specific for children and adolescents aged 2 to 20. Rather than using standard cutoff points to indicate classifications of weight, the charts for children show BMI percentiles by age in months and years. The general shape of the growth curve is similar for boys and girls, but the stages of growth, especially the timing of growth spurts, differ. BMI that plots between the 10th to the 85th percentile on the chart is described as normal. Underweight is a BMI less than the 10th percentile. Overweight plots above the 85th percentile and obese is above the 95th percentile.

Considerations

BMI is only one of several indicators and is not for use as a sole diagnostic tool. BMI for children helps frame a clinical impression of a child's health and risk of weight-related disease in combination with other diagnostic tools and health markers. Consult your pediatrician or registered dietitian if you have concerns about your child's weight or growth rate.