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Key Points

  • Microcephaly is characterized by a smaller than normal head circumference. A head circumference measurement of 3 SD below the mean gives a clinically meaningful definition because it has been associated with mental retardation.

  • Microcephaly has been associated with a number of genetic, infectious, and environmental exposures.

  • Detailed ultrasound examination to exclude other anomalies is necessary.

  • A careful history is needed when this diagnosis is suspected. Likewise, the head circumferences of the parents should be measured.

  • Little information is known about the antenatal natural history of microcephaly, especially during the first and second trimesters.

  • The diagnosis of microcephaly should not alter obstetric management.

  • Newborns with microcephaly require an extensive work-up to determine the etiology of the condition.

  • The long-term prognosis and recurrence risk for microcephaly is related to its underlying cause.


Microcephaly is characterized by a smaller-than-normal head circumference (Figures 20-1 and 20-2). The clinical significance of microcephaly is its association with a small brain— microencephaly. A difference of opinion exists as to whether the lower limit of a normal head circumference should be defined as 2 or 3 SD below the mean (Avery et al., 1972). When a head circumference of 2 SD below the mean is used to define microcephaly, the association with mental retardation is inconsistent. Using such a definition, 2.5% of the general population would be considered microcephalic, and therefore a large number of infants with normal intellectual function would be included (Martin, 1970; Sells, 1977). A head circumference measurement of 3 SD below the mean gives a more clinically meaningful definition of microcephaly, as the correlation of this measurement with mental retardation is stronger (Davies and Kirman, 1962; Warkany et al., 1981).

Figure 20-1

Axial image of fetal head showing biparietal diameter and head circumference measuring at less than the 2nd percentile for expected gestational age.

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Figure 20-2

Newborn infant with severe microcephaly. The infant’s length and weight were at 95%, but the head circumference was below 5%. The etiology for this infant’s microcephaly, which was detected prenatally, is unknown (Courtesy of John Foster.)

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The most affected part of the brain in microcephaly is the forebrain, and there is frequently associated macrogyria, pachygyria, and basal ganglia atrophy (Davies and Kirman, 1962).


Microcephaly is caused by diverse genetic and environmental factors that disturb brain growth in prenatal and early postnatal life (Warkany et al., 1981). The proportion of cases of microcephaly due to genetic causes has been estimated at 20% to 33% (Van den Bosch, 1959; Cowie, 1987). However, much of the data on which such estimates are based relate to patients born before ...

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