Children grow and develop on different timelines, but most undergo rapid growth or growth spurts at some time, according to KidsHealth.org. Two children who are the same gender, height and age may have different weights depending on where they are on the growth curve. During puberty, both boys and girls undergo hormonal changes as well as growth spurts in both height and weight.
History
Prior to 2000, BMI was not routinely used to assess the growth of children. Instead, a child's weight and height were plotted on a weight-for-stature chart. Beginning in 2000, new BMI growth charts were developed for children and adolescents ages 2 to 20 by the Centers for Disease Control and Prevention. Today, BMI is the recommended measure to determine if a child is underweight, overweight or obese.
Identification
Body mass index, or BMI, is calculated as a ratio of a child's weight and height. For most children and adolescents, the BMI is reliable as an indirect measure of body fatness. Measuring BMI is an inexpensive method to screen for weight categories related to health problems, according to the Centers for Disease Control and Prevention.
Calculation
BMI equals weight in kilograms divided by height in meters squared. If the child was measured with a standard tape measure and bathroom scales, convert to metric units to use this formula. To convert weight in pounds to kilograms, divide the number of pounds by 2.2. To convert height in feet and inches to centimeters, first convert height in feet and inches to height in inches and then multiply the number of inches by 2.54. Finally, to convert height in centimeters to height in meters squared, divide the number of centimeters by 100 to get meters and then multiply the number of meters by itself to get meters squared.
Meaning
BMI for children is plotted on a growth chart that contains percentile lines to show whether a boy or girl of a given age in years and months is underweight, healthy weight, overweight or obese. For children ages 6 to 15, select the BMI for Age chart for boys or for girls ages 2 to 20 years. A child with a healthy weight will plot above the fifth percentile and less than the 85th percentile. Above the 95th percentile is interpreted as obese and between the 85th and 95th percentiles indicates a child is overweight. Below the fifth percentile indicates a child is underweight and fewer than 5 percent of children of the same sex and age in years and months weigh less than this child.
Considerations
Although the BMI value is calculated the same for children and adults, it is interpreted differently. For children and adolescents, BMI is age and gender specific. Because of the normal growth curve for children, BMI changes rapidly and comparisons against a standard relate to other children of the same gender and age in years and months. Adult weight categories do not apply to children.
BMI is not used for children under age 2 years because of the difficulty of obtaining an accurate height measure.
BMI for age and gender is not intended for use as a sole diagnostic tool. BMI for children contributes to forming an overall clinical impression and is used in conjunction with other diagnostic tools and health markers to assess health status and disease risk.
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