CranioCalc

Calculators

Head Circumference Z-Score

Occipitofrontal head circumference compared with the WHO Child Growth Standards, birth to 5 years. The result updates as you type.

Sex
cm
weeks

If under 37 weeks, age is corrected for prematurity until 24 months of chronological age.

Method

z = (HC ÷ M − 1) ÷ S

M (median) and S (coefficient of variation) come from the WHO head circumference-for-age standards for the child's sex. The Box-Cox power L equals 1 at every age for head circumference, so the LMS method reduces to the formula above. Monthly values are interpolated linearly for ages between whole months (1 month = 30.4375 days).

Corrected age subtracts the weeks born early (40 − gestational age) from chronological age. Correction is commonly applied for infants born before 37 weeks through 2 years of age.

Microcephaly is commonly defined as a head circumference more than 2 standard deviations below the mean for age and sex (severe below −3), and macrocephaly as more than 2 above. Head circumference does not reflect head shape: children with sagittal synostosis, for example, can have a normal or large circumference.

Source: WHO Multicentre Growth Reference Study Group. WHO Child Growth Standards: Head circumference-for-age, arm circumference-for-age, triceps skinfold-for-age and subscapular skinfold-for-age. Methods and development. Geneva: World Health Organization; 2007. WHO standards.

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Enter age and head circumference
  1. < −3Severe microcephaly range< 0.1st pct
  2. −3 to < −2Microcephaly range< 2.3rd pct
  3. −2 to +2Within 2 SD of the median
  4. > +2 to +3Macrocephaly range> 97.7th pct
  5. > +3Marked macrocephaly range> 99.9th pct