Late onset microcephaly: failure of prenatal diagnosis.

We present a case of recurrent primary developmental microcephaly of late onset, the prenatal diagnosis of which could not be achieved despite performing targeted serial ultrasound scans that revealed no obvious fetal abnormality. Serial scans for head measurements and detailed examination of the brain anatomy by both transabdominal and transvaginal sonography including color and power Doppler assessment revealed no obvious brain abnormality. Frontal lobe distance and thalamic frontal lobe distance at 36 weeks were on the 30th and 50th centiles, respectively. Growth velocity remained on the 50th centile up to 36 weeks; between 36 and 38 weeks measurements were between the 35th and 40th centiles. The infant was delivered by Cesarean section at 38 weeks on parental request. On examination after birth the head circumference was on the 9th centile, but the facies was that of a microcephalic child with a sloping forehead and neurologically he was severely abnormal. The adequacy of the normal reference ranges used is reviewed: the use of sex-specific growth charts at 38 weeks would have demonstrated the biparietal diameter and the head circumference to be on the 20th and 15th centiles, respectively, rather than just below the 40th centile. However, even sex-specific charts may not allow the recognition of a substantial number of affected fetuses.