Growth charts consist of a series of percentile curves that illustrate the distribution of selected body measurements in children. They are used by pediatricians, nurses, and parents to track the growth of infants, children and adolescents. Growth charts are not intended to be used as a sole diagnostic instrument. Instead, they are tools that contribute to forming an overall clinical impression about a child's well-being.

Percentiles

Percentiles are the measurements that show where a child compared to others. On growth charts, percentiles are shown as lines drawn in curved patterns. If a child's height is at the 99th percentile, he is taller than roughly 99% of other boys his age. You can look at this another way: say you have 100 kids chosen at random in a room and you line them up in order of height. You put the tallest kids first. The boy at the 99th percentile would probably be first or second in line. Alternatively, if he's at the 10th percentile, he'll be at the back of the line, and only ~10% of boy are shorter than he is.

We wrote roughly and ~10% because growth curves are not super-exact. They can't be, because they're almost always made from a small sample of a population (think about how hard it would be to measure every child in the United States in order to make a growth curve, and then think about how hard it would be to get each measurement exactly right!).

This is one reason why it's important to use growth curves that are as reflect a child's population. For example, Dutch people are very tall and people from India tend to be shorter. If you use a Dutch growth curve for Indian kids, almost everyone would look too small!

What is a z-score?

A z-score is a simplified way of telling you how far something is from the average. A z-score of 0 is average. A z-score of 1 is above average, and a z score of 2 is well above average. In fact, a z-score of 2 is at roughly the 98th percentile). A z-score of 3 is at roughly the 99.9th percentile! Z-scores can also be negative. A score of -1 means below average (about the 16th percentile) and a z-score of -2 means roughly the 2nd percentile.

Because z-scores have a direct relationship with percentiles, a conversion can occur in either direction using a standard normal distribution table. Therefore, for every z-score there is a corresponding percentile value and vice versa.

Ideal percentile for a child

There is no ideal number. Healthy children come in all shapes and sizes, and a child at the 5th percentile can be just as healthy and intelligent and happy as a child at the 55th or the 96th percentile.

Why do we need growth charts?

Growth charts help doctors and nurses check to see if a child is growing well.

Look at any group of children, and you'll see kids of the same age in all shapes and sizes. Some kids look tiny next to their peers, while others literally stand head and shoulders above everyone around them. Each child grows in his or her own pace, and there is a wide range of healthy shapes and sizes.

Genetics, gender, nutrition, physical activity, infections and other diseases, environment, hormones, and ethnicity all influence a child's growth. Many of these factors can vary widely between families, within countries, and between countries.

Interpretation

Interpreting growth charts can be a subtle job. First and foremost, it's important to have accurate measurements. If measurements are made in a hurry or if they aren't made carefully, people may end up worrying for nothing. For example, when measuring a baby's length, it's very important to make sure that her head is against a solid surface. Laying the baby down in the middle of a table and making marks at the top of her head and soles of her feet is a good way to get a bad measurement.

When looking at a child's growth, doctors think about different things. Is a young child following roughly the same track? Or has the child's percentile score suddenly changed dramatically? Big increases in head circumference percentiles, for example, can mean there may be increased pressure inside a child's head. This might mean a percentile score of 60 at one checkup and 90 at the next. If you think you see a pattern that you aren't sure about, you should talk to your child's pediatrician.

Alternatively, growth in teenagers and adolescents may vary a lot, and kids in these groups aren't expected to follow the same percentiles (especially for height).

Sometimes doctors like to take a closer look at children who measures below the 3rd percentile or above the 97th percentile as well. Some children in these categories are just naturally big or naturally small, but a few others may have problems (such as with brain growth or increased pressure inside their heads). Head size is only one measurement, and more tests are needed to know if anything is wrong. Again, if you think something isn't right, talk to your child's pediatrician.

Why do doctors use Growth Charts?

Growth charts have now become a standard part of any checkup, and they show health care providers how the kids are growing in comparison to other children of the same age and gender. They also allow doctors to see the pattern of kids' height and weight gain over time, and whether they are developing proportionately. As we wrote in the last section sudden changes in growth percentiles might indicate a health problem.

What measurements are made?

Weight, height/length, and head circumference are measured for ages birth to 36 months. Weight, stature/height, BMI, head circumference are often measured for ages 2 to 20 years. Rarely, chest circumference, mid upper-arm circumference and skinfold thickness measurements are also done. The measurements in this last group are often done in developing nations where malnutrition is a serious problem. It's important to look at all the parameters and have a comparison made to get a full picture of a child's growth. Remember that consulting a healthcare provider is also very important.

Why is Head Circumference Measured?

In infants, head circumference (the distance around the largest part of the head) can provide clues about brain development. If a baby's head is bigger or smaller than most other kids, and or the head circumference increases drastically or stops increasing, it may indicate a problem.

For example, an unusually large head may be a sign of hydrocephalus, a buildup of fluid inside the brain. A head that's smaller than average may be a sign that the brain is not developing properly or has stopped growing.