There are no chromosomal or biochemical abnormalities that can be used for the prenatal diagnosis of primary microcephaly. However, since scalp hair directional patterning is determined between 10 and 16 weeks of fetal life, and since at least 85% of patients with primary microcephaly show aberrant scalp hair patterning, it may be concluded that a deficit of brain growth and consequently also of head size exists by the 17th week of pregnancy. Thus, ultrasonic determination of fetal head size may permit the diagnosis or exclusion of primary microcephaly in the early second trimester of pregnancy.