Microcephaly Head circumference is measured from just above the glabella (the most prominent point on the frontal bone above the root of the nose) to the most posterior prominent point of the occipital bone using a tape measure. Some standard charts are organized by centiles [Hall et al. [2007]], others by standard deviations [Farkas, [1981]]. It is important to add an indication of how far below the normal standard the head circumference is if an accurate assessment of this can be made. Microcephaly is an absolute term. The term relative microcephaly can be used when the head size centile is less than the centile for height, for example, head size at the 3rd centile with height at the 75% for age and sex. On prenatal ultrasound, microcephaly is diagnosed if the head circumference or the biparietal diameter is more than three standard deviations below the mean. Decreased size of cranium Small skull Decreased circumference of cranium UMLS:C0424688 Abnormally small head Fyler:4310 HP:0000252 Small head circumference HP:0001366 human_phenotype HP:0005489 HP:0005485 Head circumference below 2 standard deviations below the mean for age and gender. Small head SNOMEDCT_US:271611007 Abnormally small skull Reduced head circumference small cranium Decreased size of skull Abnormally small cranium small calvarium HP:0005497 Decreased size of head Aplasia/Hypoplasia of the cerebrum Decreased head circumference