The dilemma in prenatal diagnosis of idiopathic microcephaly.

It is estimated that 20 to 35 per cent of idiopathic microcephaly is hereditary. Common practice dictates that after the birth of such a case, subsequent pregnancies should be monitored ultrasonographically in order to facilitate early antenatal diagnosis, and thus genetic counselling. Two cases are reported to indicate the difficulties encountered. In case 1 head growth appeared to be normal until the 20th week of gestation and then slowed down to 31 cm at birth. In case 2 head growth proceeded normally until the 28th week of gestation; at birth it was 32 cm and over the succeeding months became markedly microcephalic. The accuracy of various ultrasonographic techniques is reviewed, and the limitations are discussed. In the light of these findings it is concluded that reliable prenatal diagnosis of hereditary microcephaly is not available as yet.