WHO Growth Charts Explained

If you've been to a child health clinic, you've seen the chart: a page of sweeping curves with your child's measurement plotted as a dot. Those curves come from the WHO Child Growth Standards, and understanding what they show — and what they don't — takes a lot of anxiety out of the appointment.

What makes the WHO standards different

Most growth charts before 2006 were descriptive: they recorded how children in a particular place and period actually grew, whatever their feeding or health circumstances. The WHO standards, released in 2006, are prescriptive — they describe how children should grow under optimal conditions.

They came from the Multicentre Growth Reference Study, which followed children across six countries — Brazil, Ghana, India, Norway, Oman, and the USA — selected for favourable conditions: breastfeeding, good nutrition, healthcare access, and non-smoking mothers. A central finding justified the whole approach: under those conditions, young children's growth patterns were remarkably similar across all six populations. Ethnicity mattered far less than nutrition and health, which is why one international standard is defensible for children under five.

The four charts and what each is for

One important measurement detail: children under two are measured lying down (length), and those over two standing (height). Length runs roughly 0.7 cm greater than height on the same child, and the charts account for the switch — which is why the correct chart for the child's age matters.

Reading percentiles

A percentile tells you where your child falls among 100 children of the same age and sex. A boy on the 40th percentile for weight weighs more than 40 of every 100 boys his age and less than 60 of them. That's all it means — it isn't a score, and higher isn't better.

• The 50th percentile is the median, not a target. Half of all healthy children are below it by definition.
• The healthy range is wide. Anywhere roughly between the 3rd and 97th percentile is generally considered within normal variation.
• The trend beats the number. A child steadily tracking the 15th percentile is usually less concerning than one who drops from the 75th to the 25th over a few months. Clinicians watch for crossing percentile lines.

Z-scores: what clinicians actually use

WHO charts are built on z-scores (standard deviations from the median), which handle the extremes better than percentiles. A z-score of 0 is exactly the median; +1 is one standard deviation above; −2 is two below. Rough correspondences:

In WHO terminology, weight-for-height below −2 z indicates wasting; height-for-age below −2 indicates stunting; weight-for-age below −2 indicates underweight. These thresholds drive global nutrition monitoring.

Why exact age matters more than you'd think

Growth is so rapid in early childhood that plotting against the wrong age can shift a child's apparent percentile substantially — the difference between 5 months and 6 months of age is meaningful on a weight-for-age curve. Clinics therefore calculate age precisely from the date of birth, in completed months and days, rather than rounding.

Two adjustments matter here. Premature babies are plotted using corrected age — chronological age minus the weeks born early — typically until around age two, so a baby born three months early is assessed at 9 months corrected when 12 months chronologically. And when moving between charts, the WHO standards cover 0–5 years; many countries switch to national or CDC references for children over five.

You can get a child's exact age in years, months, and days from a date of birth with our age calculator — or read how the arithmetic works in How Age is Calculated.

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