By Erica Lesperance, RD, LD

Parents often talk in terms of “percentiles” when discussing the height and weight of their children. You may have heard a mother say something like, “Johnny is in the 95^th percentile for height,” meaning that he is as tall, or taller than 95% of children his age across the United States. This information comes from plotting Johnny’s height on a growth chart. The original growth charts were developed by pediatric health care providers in 1977 and adopted by the World Health Organization for international use in 1978. Today, they continue to be the most widely used tools to track growth and development in children.

While the original growth charts are still useful, a changing population has led to recent modifications of growth charts to more accurately reflect the nation's cultural and ethnic diversity. Charts for infants have improved significantly due to new data and better statistical procedures. In addition to the revised charts, new charts for assessing a child's body mass index, or BMI, were added to help physicians and other health care providers detect early signs of weight problems.

With childhood obesity reaching epidemic proportions, there has been increasing focus on the usefulness of growth charts in determining presence of, or even risk for, becoming overweight. Your pediatrician should be following the growth of your child closely using an updated growth chart. However, as a parent you should also learn to use and understand these tools that can provide valuable information about your child’s growth status. Here are some topics to understand when deciphering your child’s growth chart.

Finding the right growth chart

There are different growth charts for males and females (scroll down the page to download the appropriate PDF file). When they’re printed, they are often blue and pink, respectively. They are divided by age, the first being from birth to 36 months (in one-month increments) and the second from two to 20 years (in one-year increments). Once you have downloaded the correct growth chart, gather your information—your “data points”—by measuring your child’s height and weight.

Gathering your data points

To plot your child’s current weight, find his age across the bottom of the sheet. It may help to draw a vertical line up from that age to make it easier to see—use a pencil so you can erase the line easily. Now find your child’s weight on the right hand side of the chart, and again if you wish, draw a straight line across the page to the left side at that point. Find the spot where the two lines intersect, and draw a dot in that place. The dot will be surrounded by several curved lines. Choose the line that is closest to your dot and follow it to the right until you find a number such as 5, 10, 25, 50, 75 or 90. This number is your child’s percentile for weight. If your dot falls between two curves, say between the 50 and 75, then your child’s percentile is somewhere between 50 and 75, and you can estimate based on how close he falls to one or the other. For example, if his dot falls is between 50 and 75 but almost all the way to 75, you might estimate that he is in the 70^th percentile. You can plot your child’s height in the same fashion on a height chart (called length-for-age or stature-for-age depending on your child’s age). (If you want further instructions on using the charts, you can view a very helpful PowerPoint presentation here.)

Understanding percentile curves

So now that you have a percentile for your child’s weight and height, what does that mean? The percentile curves on these charts represent the percentage of children that are of the same height or weight. The 50th percentile means that 50% of children will be above this point and 50% will be below it. While this information is important, how your child has been growing can be more important than what percentile he is in. Children at or below the 5th percentile for weight may be normal if the rate at which they are growing is normal. They might just be small. But if a child is growing steadily at the 5^th percentile and then relatively quickly increases to the 50^th, this could indicate a problem. But keep in mind that between birth and eighteen months of age, children are growing very rapidly and often change percentiles. After this age, children usually follow their growth curves fairly closely. It is always important to look at a number of different values of height and weight over time to figure out a child's rate of development. If you have concerns about your changes in your child’s percentiles, discuss this with your pediatrician.

BMI-for-age

It is important to consider how much a child weighs in relation to how tall he is. The data points on the weight and height charts alone do not tell the whole story. Until recently, the 1977 weight-for-stature (or weight-for-height) charts were used to plot both of these points on one chart. However, BMI has since become the recommended measure to determine if children over two years old are overweight, and therefore the old charts were replaced with the BMI-for-age charts. BMI is calculated from weight and height measurements, resulting in a single number that is used to judge whether an individual’s weight is appropriate for his height. If you do not know your child’s BMI, ask his doctor, or calculate it yourself with this BMI calculator for children and teens. You can then plot that number on the BMI-for-age chart.

Each of the BMI-for-age charts (one for boys, one for girls) contains a series of curved lines indicating specific percentiles, just as you find in the height and weight charts. It is important to note that a child's BMI is expected to decrease during the preschool years, then increase into adulthood. The percentile curves show this pattern of growth.

Stay informed

By keeping track of your child’s weight, height, and BMI along with your pediatrician, you can stay informed about your child’s growth status. Generally, a BMI above the 85^th percentile means a child is at risk of overweight, while a BMI at or above the 95^th percentile is considered overweight. If your child’s BMI is approaching these levels, be sure to discuss your child’s unique situation with your pediatrician, and together you can determine if action is necessary. Keep in mind that growth charts are an important tool for monitoring children's development, but they are just one of the tools used to ensure a child is growing and developing normally.

For more information about growth charts, visit the National Center for Health Statistics website.