AbstractMicrocephaly (small head) is clinically important only if there is concomitant micrencephaly (small brain). Extensive studies on patients in mental institutions have shown that there is close correlation among microcephaly, micrencephaly, and mental retardation when the head is more than three standard deviations below the norm. If the small head is less than two standard deviations below the norm, no strong correlation exists with either small brain or mental retardation. High‐resolution ultrasound permits imaging of the fetal head in utero, allowing accurate evaluation of head size and detection of intracranial anomalies. The microcephalics detected in utero over a 2‐year period form the basis of this series, showing close correlation with the known clinical data on children with microcephaly. An approach to ultrasound detection and evaluation of fetal microcephaly is propo