Small head size (microcephaly) is most often defined as more than two standard deviations below the mean, which corresponds to the 2.3 percentile. Some authors prefer the cutoff of 3 standard deviations below the mean. WIC uses the risk criterion of the 5th percentile. The growth chart most commonly used for children over age 3 years (see Nellhaus) shows standard deviations rather than percentiles. For more information about the use of standard deviations click here.
In some circumstances,
microcephaly may be expected, especially with very short children; it
can also be due to many familial, prenatal, and postnatal causes, including
prenatal and postnatal severe malnutrition. Depending on head size at
birth, microcephaly may be classified as congenital
the two types tending to have different causes.