The WHO boys growth chart is a set of standardized curves that show how healthy boys typically grow from birth to age 5. Pediatricians use these charts to plot a child’s weight, length or height, and head circumference over time, comparing each measurement against established percentiles. Rather than defining a single “normal” size, the charts show the full range of healthy growth so parents and doctors can spot potential problems early.
What the WHO Growth Charts Measure
The WHO provides several different charts for boys, each tracking a different measurement against age. The main ones are weight-for-age, length-for-age (called height-for-age after age 2), weight-for-length, head circumference-for-age, and BMI-for-age. Together, these give a complete picture of whether a boy is growing proportionally or whether one measurement is lagging behind or pulling ahead of the others.
Each chart displays a series of curved lines representing percentiles: the 3rd, 15th, 50th, 85th, and 97th. The 50th percentile is the median, meaning half of healthy boys measure above that line and half below. A boy on the 15th percentile for weight isn’t necessarily underweight. He’s simply lighter than about 85% of boys his age, which can be perfectly normal if he’s been tracking along that curve consistently.
How the Charts Were Created
The WHO Multicentre Growth Reference Study collected data from roughly 8,500 children across six countries: Brazil, Ghana, India, Norway, Oman, and the United States. This wasn’t a random sample. The researchers deliberately selected children who were raised under optimal conditions, including breastfeeding as the primary source of nutrition, nonsmoking mothers, and adequate healthcare. The goal was to capture how children grow when nothing is holding them back, making the charts a standard for what growth should look like rather than a reference for what it happens to look like in a given population.
Because the study included children from diverse ethnic and cultural backgrounds, the resulting charts apply internationally. The breastfed infant is the baseline model, which means formula-fed babies may appear to grow slightly faster in the first year compared to the chart’s median. That difference is expected and reflects the growth pattern of breastfed children, not a shortcoming of formula feeding.
Understanding Percentiles and Z-Scores
Percentiles are the simplest way to read a growth chart. If your son is at the 75th percentile for height, he’s taller than about 75% of boys his age. Doctors sometimes use Z-scores instead, which express the same information as standard deviations above or below the median. A Z-score of 0 equals the 50th percentile. A Z-score of +1 is roughly the 84th percentile, and a Z-score of -1 is roughly the 16th percentile.
The key conversions that matter clinically:
- Z-score of -2 (3rd percentile): the threshold below which doctors investigate for possible growth problems
- Z-score of -3 (0.1 percentile): suggests severe growth restriction
- Z-score of +2 (97th percentile): the upper boundary of typical growth
- Z-score of +3 (99.9 percentile): may indicate a condition causing excessive growth
A single measurement at a low or high percentile is rarely concerning on its own. What matters more is the trend. A boy who has always tracked along the 10th percentile is likely just small for his age. A boy who drops from the 50th percentile to the 10th over a few months is showing a change in growth velocity, which warrants a closer look.
WHO Charts vs. CDC Charts
In the United States, both WHO and CDC growth charts exist, and they serve slightly different purposes. The CDC and the American Academy of Pediatrics recommend using WHO charts for all children from birth to age 2, regardless of whether they’re breastfed or formula-fed. For children aged 2 to 20, CDC growth reference charts are recommended instead.
The practical reason for this split is data quality. The WHO charts were built from longitudinal measurements, meaning the same children were weighed and measured repeatedly at frequent intervals during infancy. The CDC charts, by contrast, are based on cross-sectional data, and the sample sizes for infants under 6 months were small. Weight data for newborns to 2 months weren’t even available in the CDC dataset. That makes WHO charts more reliable for the youngest age group.
For boys aged 2 to 5, there’s overlap. The WHO charts technically cover birth to 5 years, but the CDC notes that for BMI-for-age in this range, the WHO charts offer little advantage over the CDC versions. Since CDC charts extend all the way to age 20, most U.S. providers switch to them at age 2 for continuity.
What the Charts Flag in Boys
Growth charts aren’t diagnostic tools. They’re screening tools that help identify children who may need further evaluation. The WHO uses specific Z-score cutoffs to define growth concerns in children under 5:
Wasting refers to a child whose weight is too low for his height. It’s defined as a weight-for-height Z-score below -2, meaning the child falls below the 3rd percentile for weight relative to his length. Severe wasting is a Z-score below -3. This typically signals acute malnutrition or illness rather than a child who is naturally lean.
Stunting refers to a child whose height is too low for his age, reflecting chronic undernutrition or repeated illness over time. It follows the same thresholds: a height-for-age Z-score below -2 for moderate stunting and below -3 for severe stunting.
On the opposite end, a weight-for-height Z-score above +2 is classified as overweight, and above +3 as obese. These cutoffs help pediatricians identify early patterns that could affect a child’s long-term health, even at very young ages.
How to Read Your Son’s Growth Chart
At each well-child visit, your pediatrician will plot your son’s measurements on the appropriate chart. You’ll typically see dots connected by a line showing his growth trajectory over time. The most important thing to look at isn’t where the dot falls on a single visit. It’s the shape of the line across multiple visits.
A line that follows roughly parallel to the printed percentile curves, even if it sits near the bottom or top of the chart, generally indicates healthy growth. A line that crosses two or more major percentile lines in either direction suggests a change worth investigating. Crossing upward might mean excess weight gain, while crossing downward could point to feeding difficulties, food sensitivities, or an underlying health issue.
Growth in boys isn’t perfectly smooth. Infants often grow in spurts, and it’s common for a boy’s percentile to shift somewhat during the first two years as he settles into his own genetic trajectory. A baby born large may gradually move toward a lower percentile if his parents are average-sized, and that’s normal. The pattern typically stabilizes by age 2, after which most children track fairly consistently along their established curve through early childhood.
Where to Find the Charts
The WHO publishes free, downloadable growth charts on its website in both percentile and Z-score formats. Separate charts exist for boys and girls, since growth patterns differ between them even in infancy. Charts are available for birth to 6 months, birth to 2 years, and birth to 5 years, letting you choose the version that gives the most detail for your child’s age. Your pediatrician’s office will typically have these built into their electronic health records, so your son’s measurements are plotted automatically at each visit.