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What is a percentile?

We get a lot of questions about how to interpret the growth charts. Some common questions are:

  • How can I see how my child's growth compares to an average child's growth?
  • What do the colored lines on the chart mean?
  • How do I choose which growth standard to use? Why do they all give different percentiles?

This guide is intended to answer these common questions and more.

What a percentile isn't

There can be a lot of confusion, emotion, and worry among parents about percentiles. You may see some parents boasting on social media about how proud they are that their little munchkin is in the 90th percentile for weight, but you seldom see parents boasting about their child being in the 10th percentile.

A percentile is not a contest! This is not a test where the goal is to get a score of 100%.

The idea that a child in the 10th percentile is something to be ashamed of while a child in the 90th percentile is something to be proud of is completely misguided. Both children may be perfectly healthy (or unhealthy) - they're just different sizes! Going forward, remember just one thing: the number doesn't matter - it's not a contest.

Percentiles and statistics

So then what is a percentile? It's just statistics. Each of the growth charts you can choose from (e.g. WHO, CDC, etc...) was built based on a reference set of measurements from thousands of children. From that data, researchers determined the average child's measurement at different ages. Normal, healthy children are not all exactly the same size though, so they also determined the spread of measurements around that average (the standard deviation and skewness).

What does all that mean? If you measure 1000 babies with an average weight of 4 kg, very few of them will actually weigh exactly 4 kg - some will weight more and others will weigh less. If the standard deviation of that measurement is 0.5 kg, that means that for a normal distribution, 950 of them weigh between 3 and 5 kg (two standard deviations above and below the average), and 25 will be below 3 kg and the other 25 will be above 5 kg.

A percentile just tells you where a given measurement falls in that distribution. A percentile of 50% means the measurement is exactly in the middle, so it is right on the average. In the example above, a measurement of 3 kg would be at 2.5%, since 2.5% of the measurements are below that and 97.5% are above that. Likewise a measurement of 5 kg would be at 97.5%.

Remember, the growth references are constructed using measurements from a large sample of healthy children. This means:

  • Half of the healthy children measured were below the 50th percentile.
  • 1 in 4 of the healthy children measured were below the 25th percentile.
  • 1 in 10 of the healthy children measured were below the 10th percentile.
  • 1 in 20 of the healthy children measured were below the 5th percentile.

So which growth curve should I use?

For breastfed babies, the WHO curves are usually the best choice. Some groups of people may be on average larger or smaller than the WHO reference population, but remember, the exact percentile number doesn't matter! The app also has dozens of country-specific growth references you can choose from. You'll get a different percentile number depending on which chart you choose, but the overall growth trends should still be similar. As your child gets older you may want to switch from the WHO curve to the country-specific curve. Different countries have different recommendations for the age to switch charts, but it usually ranges from 2-5 years old.

How do I read the chart? What do the colored lines mean?

The colored lines on the chart are lines at a constant percentile. Each line shows how a child would grow if they stayed at that exact percentile. Children don't stay at constant percentiles though, they drift up and down in percentile and that's ok! The point of the colored lines is to show the average growth trend. If your child is staying near the same set of lines on the chart they're growing at a normal rate. It doesn't matter whether it's the 10-25% range or the 75-90% range.