If you've ever entered your child's measurements into a weight-for-age calculator or height-for-age calculator and gotten two different percentiles — one from the CDC and one from the WHO — you're not seeing an error. The two chart systems are built from different data, answer slightly different questions, and are designed for different age ranges. Understanding the difference helps you interpret your child's results correctly.
The Short Answer
The American Academy of Pediatrics (AAP) recommends:
- Birth to 24 months: Use WHO growth standards.
- Ages 2 to 20 years: Use CDC growth charts.
Most US pediatricians follow this recommendation. If your child is under 2, the WHO chart is the primary reference. If they're 2 or older, the CDC chart takes over. This is exactly how our growth calculators work — they show both when available but clearly mark which one is recommended for your child's age.
Side-by-Side Comparison
Here is a detailed comparison of the two growth chart systems:
| Feature | WHO Child Growth Standards | CDC Growth Charts |
|---|---|---|
| Year published | 2006 | 2000 |
| Age range | Birth to 5 years | Birth to 20 years |
| Type | Prescriptive standard (how children should grow) | Descriptive reference (how children did grow) |
| Population | ~8,500 healthy breastfed children from 6 countries (Brazil, Ghana, India, Norway, Oman, USA) | US national survey data from NHES and NHANES (1963–1994) |
| Feeding method | Breastfed (at least 4 months exclusive, continued to 12 months) | Mix of breastfed and formula-fed |
| Study design | Longitudinal (followed same children over time) + cross-sectional | Cross-sectional surveys (different children at different ages) |
| Environment | Optimal conditions: non-smoking mothers, adequate nutrition, healthcare access | General US population (all socioeconomic levels) |
| Statistical method | LMS (Box-Cox power exponential) | LMS (same method) |
| Charts available | Weight-for-age, length/height-for-age, weight-for-length/height, BMI-for-age, head circumference-for-age | Weight-for-age, stature-for-age, weight-for-stature, BMI-for-age, head circumference-for-age |
| BMI available from | Birth (but not clinically used before age 2) | Age 2 years |
| Recommended by AAP for | Children under 24 months | Children ages 2–20 years |
Why the Percentiles Differ
The core reason the two charts give different percentiles is that they describe different populations. The WHO standards are based on children raised in optimal conditions — breastfed, non-smoking households, adequate healthcare. The CDC charts include the full range of the US population, including formula-fed infants and children from all socioeconomic backgrounds.
In practice, this means:
- Breastfed infants tend to track higher on the WHO chart in the first 6 months, because the WHO reference population was breastfed. On the CDC chart, these same infants may look slightly below average because the CDC reference includes formula-fed babies, who tend to gain weight faster in early infancy.
- After 6 months, breastfed infants may appear to "drop" on the CDC chart — not because they're growing poorly, but because formula-fed infants in the reference population gain weight more rapidly. On the WHO chart, the same infant would track steadily.
- Differences of 5–15 percentile points are common and do not indicate a problem. Both results are correct relative to their own reference population.
What "Prescriptive" vs. "Descriptive" Means
This is the most important conceptual difference between the two chart systems:
- The WHO standards are prescriptive — they define how children should grow when raised in healthy environments. They set a target. A child below the 3rd percentile on the WHO chart may genuinely be growing sub-optimally.
- The CDC charts are descriptive — they document how American children actually grew during a specific historical period. They're a statistical reference, not an ideal. A child below the 3rd percentile on the CDC chart is simply smaller than 97% of the US reference population — which may or may not indicate a problem.
This is why the WHO chart is preferred for infants: it's better at identifying true growth faltering because its reference group consisted of optimally growing children. If a breastfed infant looks below average on a chart built from formula-fed babies (CDC), you might investigate unnecessarily.
When Both Charts Are Shown
On GrowthPercentile.com, when both data sources are available for your child's age, both percentiles are displayed. The recommended chart for your child's age range is clearly labeled. This lets you:
- See the result your pediatrician would use (the recommended chart)
- Compare against the other reference population for context
- Understand why results might differ from other calculators that use only one chart system
Try it yourself with the weight-for-age calculator for an infant under 2 — you'll see both WHO and CDC results side by side.
International Use
Outside the United States, the WHO growth standards are the primary reference in most countries. The World Health Organization recommends their charts for all children worldwide. Over 140 countries have adopted the WHO standards for clinical use.
In the US, the hybrid approach (WHO for < 2, CDC for 2–20) was adopted in 2010 and is endorsed by the AAP, the CDC itself, and most major pediatric health organizations.
Does It Matter Which Chart I Use?
For most children, both charts will tell a similar story. Where it matters most:
- Infants under 12 months: Use the WHO chart. It avoids false "faltering growth" alarms for breastfed babies.
- Children over 5: Only CDC charts are available (WHO stops at age 5).
- BMI assessment: Only CDC BMI-for-age charts are recommended clinically, and only from age 2.
- Consistency: Always compare measurements taken at different ages against the same chart system to see meaningful trends.
If you're unsure which percentile to focus on, follow the AAP recommendation: WHO under 2, CDC from 2 onward. And when in doubt, bring the chart to your next well-child visit — your pediatrician can put the numbers in context. For a deeper look at how percentiles work and what the numbers mean, see our guide: What Is a Growth Percentile?