Calculate percentiles and Z-scores using WHO (0-24m), CDC (2-20y), or Fenton 2025 (preterm) standards.
A pediatric growth chart is a clinical tool used to monitor a child's physical development over time. Growth charts plot a child's weight, length/height, head circumference, and BMI against age-specific reference populations. Healthcare providers use them to identify potential growth disorders, nutritional problems, and chronic diseases early.
The two most widely used standards are the WHO Child Growth Standards (recommended for children birth to 24 months) and the CDC Growth Charts (recommended for children 2 to 20 years). Preterm infants use the Fenton Growth Charts until they reach term-equivalent age.
Growth percentiles and Z-scores are calculated using the LMS method (Lambda-Mu-Sigma), a statistical parameterization developed for growth reference data. The formula is: Z = ((value/M)^L - 1) / (L × S), where L is the Box-Cox power, M is the median, and S is the coefficient of variation at a given age and sex.
The Z-score represents how many standard deviations a measurement is from the median. The percentile is derived from the Z-score using the standard normal distribution. For example, a Z-score of 0 equals the 50th percentile (median), and a Z-score of -2 is approximately the 2nd percentile.
The WHO growth charts (2006) describe how healthy breastfed children should grow under optimal conditions. They are based on a multinational study of children raised in ideal health environments. The CDC recommends using WHO charts for children birth to 24 months.
The CDC growth charts (2000) describe how U.S. children did grow, based on national survey data from the 1960s-1990s. They are recommended for children 2 to 20 years. Key difference: WHO charts tend to show higher breastfed infant weights and may classify fewer infants as underweight compared to CDC charts.
The Fenton preterm growth charts should be used for infants born before 37 weeks gestational age. These charts cover 22 to 50 weeks gestational age and include weight, length, and head circumference percentiles specifically calibrated for premature infants.
After reaching 50 weeks post-menstrual age (approximately term + 10 weeks), providers typically transition to WHO growth charts using corrected (adjusted) age. Corrected age accounts for prematurity: Corrected age = Chronological age - (40 weeks - GA at birth). Most pediatricians correct for prematurity until 24 months for weight/height and 18 months for head circumference.
Being at the 5th percentile for weight means the child weighs more than 5% of children the same age and sex, and less than 95%. While this is below average, a single percentile reading is less important than the growth trend over time. A child consistently tracking along the 5th percentile is likely healthy; a child dropping from the 50th to the 5th percentile may warrant evaluation.
Healthcare providers watch for growth faltering (crossing two major percentile lines downward), failure to thrive (weight-for-age below the 2nd percentile or weight-for-length below the 5th), and disproportionate growth patterns. Z-scores below -2 or above +2 generally warrant clinical evaluation.
BMI (Body Mass Index) in children is calculated the same way as adults: weight (kg) / height (m)². However, unlike adults, pediatric BMI must be interpreted using age-and-sex-specific percentiles because body composition changes dramatically during growth.
CDC BMI-for-age interpretation: Underweight = below 5th percentile; Normal weight = 5th to 84th percentile; Overweight = 85th to 94th percentile; Obesity = 95th percentile or above. BMI-for-age is most useful for children aged 2-20 years. For children under 2, weight-for-length is used instead.
Head circumference (HC) is a proxy measurement for brain growth and is routinely measured from birth through 24 months (and up to 36 months in some practices). HC is plotted on WHO or CDC growth charts to track brain development over time.
Concerning findings include: Microcephaly (HC below the 3rd percentile or more than 2 SD below mean), which may indicate genetic conditions or congenital infections; Macrocephaly (HC above the 97th percentile), which may indicate hydrocephalus or familial large head; and rapidly crossing percentile lines, which may suggest increased intracranial pressure. Serial measurements are more informative than single readings.