WHO vs CDC: Which Chart?
WHO charts (2006) come from kids who were exclusively breastfed across six countries. They're prescriptive, what optimal growth looks like.
CDC charts are descriptive, they're how American kids actually grow, mix of breast and formula.
The practical difference is important: WHO charts show breastfed 6-month-olds heavier than CDC charts. Plotting a baby on CDC charts may indicate growth faltering; on WHO, the same infant appears appropriate. Using the wrong reference can lead to unnecessary investigation for nonexistent pathology.
Use the WHO Growth Charts and CDC Growth Charts tools to keep yourself honest.
Practical Rules
Use WHO for infants 0-24 months, especially if breastfed. After 24 months, CDC charts are appropriate and align with US population data. Some practices use a hybrid approach: WHO until age 2, then transition to CDC. Whichever system is selected should be used consistently throughout follow-up. Switching references mid-follow-up creates the appearance of sudden percentile decline in a normal child, potentially prompting unnecessary investigation.
When Percentile Crossing Matters
A kid dropping from the 90th to the 75th percentile, is that concerning or normal? Depends.
In the first 6-12 months, normal infants cross percentiles frequently, particularly during regression toward their genetic growth potential. An infant born large due to maternal diabetes may track the 95th percentile at birth but settle to the 50th percentile by 6 months of age; this is normal. Findings that warrant concern include persistent downward crossing, large percentile drops (such as falling from 75th to 10th percentile in 3 months), or declining height velocity, which may indicate inadequate nutritional intake, malabsorption, endocrine dysfunction, or systemic disease.
The key: look at velocity, not just the percentile. Consistent slow velocity is normal variation. Decelerating velocity is a problem.
Preterm Infants: Age Correction
A 3-month-old born at 26 weeks isn't really 3 months old, corrected age is what matters.
Corrected age (months) = Chronologic age (months) – (40 – gestational age in weeks) / 4.33
That 3-month-old 26-weeker is actually 6 weeks corrected age. Plot them on a term infant chart and they look tiny. Use the Fenton Preterm calculator instead and they look appropriate for 6 weeks postmenstrual age.
Correct until 2-3 years; by then genetic and environmental factors have mostly stabilized them into their growth channel.
Genetic Syndromes: Use Condition-Specific Charts
A kid with Down syndrome is going to be short. Plot them on standard charts and they look like they have failure to thrive. Use the Down Syndrome Growth charts and they're tracking fine.
Same with Turner syndrome, achondroplasia, Noonan, Prader-Willi. These kids have built-in short stature; you can't judge them against the standard population.
Use the Down Syndrome Growth and Turner Syndrome Growth calculators for those conditions. You can still compare to general population to catch unexpected growth problems, but the condition-specific reference is your baseline.
Growth Velocity: The Real Signal
Percentile position is static. Velocity is what shows you if something's going wrong.
Normal infants grow approximately 25 cm/year in length during the first year (fastest in the first 6 months) and gain about 6–7 kg in the first year. Toddlers (1–3 years) slow to approximately 8–12 cm/year and gain 2–3 kg/year. School-age children (4–puberty): 5–7 cm/year height, 2–3 kg/year weight. During the pubertal growth spurt, velocity increases to 8–14 cm/year.
A child growing consistently at 3 cm/year and tracking the 10th percentile is likely normal, representing their genetic potential. By contrast, a child previously growing 8 cm/year who decelerates to 3 cm/year represents a significant problem. Even if the child remains at the 25th percentile, the deceleration in velocity is the critical red flag. Use the Growth Velocity calculator to track height velocity accurately.
In toddlers, if weight velocity crashes but height holds steady, think poor intake or malabsorption. If height velocity crashes, think systemic illness or endocrine problems.
Bottom Line
Get the chart right, WHO for infants, CDC after 2 years, Fenton for preterm, condition-specific for syndromes.
Track velocity, not just percentile. Measurements every 1-3 months let you see the trajectory.
When something looks off, calculate the growth velocity. It'll tell you if this is normal variation or a real problem.