Understanding Growth Percentiles

A growth percentile compares one measurement with children of the same age and sex. If your baby is at the 40th percentile for weight, about 40 out of 100 babies the same age weigh less and about 60 weigh more. The 50th percentile is the median: half of children are above it and half are below.

A percentile is not a score. A baby at the 10th percentile is not doing worse than a baby at the 90th; they are simply smaller. Healthy children grow along every part of the chart, mostly because of the size of their parents.

What range is normal? Growth charts draw curves from the 3rd to the 97th percentile, and most children fall in that band. For children under 2, the CDC suggests the WHO charts and uses the 2.3rd and 97.7th percentiles (two standard deviations from the median) as the points where a doctor takes a closer look. Being outside that band is a reason to look more carefully, not a diagnosis.

Percentiles and z-scores say the same thing in different units. A z-score of 0 is the 50th percentile, −1 is about the 16th, +1 about the 84th, −2 about the 2nd and +2 about the 98th. Doctors often prefer z-scores for children far from the median because percentiles bunch up near 0 and 100.

The trend matters more than any single number. A baby who stays near the 15th percentile from visit to visit is growing steadily. A curve that crosses two of the main percentile lines (for example from above the 50th to below the 15th) over a few months is the kind of change pediatricians want to know about, especially if weight and length move differently.

Single measurements are noisy. A wriggly baby, a wet diaper, a different scale or a tape placed slightly differently can move a result by several percentile points. If a number surprises you, measure again before reading too much into it.

Weight, length and head circumference each have their own percentile, and they do not need to match. A long, lean baby might be at the 80th for length and the 40th for weight. Doctors look at how the three move together over time.

Babies born early are usually plotted at their corrected age (age from birth minus the weeks they were born early) until about age 2. Using actual age can make a preterm baby look smaller than they are.

You can check any measurement with the free Growthling percentile calculator, or log measurements in Growthling to see the whole curve on the WHO or CDC chart.

Sources: WHO Child Growth Standards; CDC, “Use of World Health Organization and CDC Growth Charts for Children Aged 0–59 Months in the United States” (MMWR, 2010). This article is general information, not medical advice.