Growth percentiles explained: what the number means

What the number means

Imagine lining up 100 girls who are exactly 6 months old, from the lightest to the heaviest. If your daughter would stand in place 46, she is at about the 46th percentile for weight: about 46 of the 100 weigh less than she does, and about 53 weigh more.

The "100 children" are not a random group from your town. They are the reference population behind the chart. For children under 2 in the US that is the WHO Child Growth Standards; from 2 to 20 it is the CDC 2000 growth charts. ¹ That is why the same weight can give two different percentiles on two different charts, as the guide on WHO versus CDC charts explains.

Percentile, z-score and the curves on the chart

Growth charts are built with a method called LMS. For every age (or length) the tables give three numbers: L, a skew adjustment; M, the median; and S, a measure of spread. From those three numbers a measurement can be turned into a z-score, the number of standard deviations from the median, and the z-score can be turned into a percentile. ²

The curves printed on a chart are simply the measurements at those percentiles, age by age. The shaded bands on childow's charts show the 25th to 75th (the middle half), the 10th to 90th, and the outer lines.

CDC recommends using the 2.3rd and 97.7th percentiles (two standard deviations either side of the median) as the outer cutoffs on WHO charts for children under 2, rather than the 5th and 95th used traditionally on CDC charts. Those cutoffs, CDC notes, are statistical conventions and not based on health outcomes. ¹

Why it is not a grade

It is easy to read the 90th percentile as "doing well" and the 10th as "doing badly". Growth charts do not work like that. Children come in different sizes for many reasons: their parents' sizes, how they were fed, whether they were born early, and plain variation. A baby who has followed the 15th percentile line since birth is growing along her own curve, just as a baby on the 85th is.

Percentiles also do not combine into an overall score. A child can be at the 80th percentile for length and the 30th for weight. That is a long, lean child, not a contradiction. Weight-for-length, a separate chart that compares weight with other children of the same length, is the way to look at those two together before age 2. From 2, CDC uses BMI-for-age instead.

Very high and very low numbers

Near the top and bottom of a chart, percentiles squeeze together. Going from the 97th to the 99th percentile takes a much bigger change in weight than going from the 49th to the 51st. For that reason clinicians often use z-scores, rather than percentiles, at the extremes.

For BMI, CDC published extended charts in 2022 for children and teens at or above the 95th percentile, because the original 2000 curves were not designed to describe very high BMIs. childow uses that extended method above the 95th percentile. ³

Very extreme values on any chart are much more often a typing slip than a real measurement: kilograms entered as pounds, a misplaced decimal point, inches entered as centimetres. childow asks you to check a number like that before it plots anything.

What patterns are worth a question

CDC's guidance is to look at growth over time, using several measurements together with the child's health and family history. ^1,4 In practice these are reasonable reasons to ask your pediatrician at the next visit, or sooner if you are worried:

• a measurement crosses two or more of the major percentile lines (for example from above the 75th to below the 25th) between visits;
• a point is below the 2nd or above the 98th percentile on the WHO chart (3rd or 97th on CDC charts after 2);
• weight and length are moving in very different directions;
• anything about eating, energy, sleep or development that worries you, whatever the percentile.

None of these is a diagnosis. Each is a question worth asking. The guide on crossing percentile lines goes through the first one in more detail.

How to get a percentile you can trust

1. Use the right chart for the age. WHO before 2, CDC from 2, in the United States. ¹
2. Use the exact age. A baby measured at 6 months and 20 days is not "6 months". childow computes age in days from the birth date and the measurement date.
3. Use corrected age if your baby was born before 37 weeks, for the first 2 years.
4. Measure the same way each time. The same scale, similar clothing, lying length under 2 and standing height from 2.
5. Keep the history. One dot tells you a position. Several dots tell you a pattern, which is what pediatricians actually use.

How childow computes percentiles

childow computes every percentile with the LMS method from the published WHO and CDC tables, which are mirrored with checksums. The engine is tested against the percentile columns in the same CDC files at hundreds of points, to within 0.01 of a unit. The sentence beside every number, "out of 100 girls this age, about N weigh less", is generated from the computed percentile, which is shown rounded and limited to the range 1 to 99.