Our objective was to determine whether the diagnosis of microcephaly present at birth is apparent using standard biometry in the second trimester. Fetuses with prenatally suspected microcephaly (biparietal diameter > or = 3 standard deviations below mean) who had a first sonogram prior to 22 weeks' gestation and a confirmation of microcephaly after birth were included in the study. We excluded all fetuses who had neural tube defects or other major associated abnormality that would lead to a suspicion of microcephaly. We therefore included fetuses who either had normal-appearing brains sonographically or intracranial calcifications as the only sonographic abnormality seen prior to 22 weeks' gestation. Seven fetuses met these criteria. One fetus was diagnosed as having microcephaly prior to 22 weeks' gestation. The other six fetuses had a normal head size prior to 22 weeks' gestation and were diagnosed as having microcephaly at 27 weeks' gestation and later. Only one of the seven fetuses had a karyotypic abnormality. We conclude that the prenatal diagnosis of microcephaly is not excluded by normal biometry on second trimester sonography.