It is necessary to think about the frequency of mistakes made at term as well as about the rare case of antenatal microcephaly when confronted with an abnormally small biparietal diameter. The authors, when confronted on twelve occasions with the problem of antenatal microcephaly, classified two situations: the discovery from systematic screening of a break in the growth of the cranial diameters. In the case of overall growth retardation of the fetus it becomes necessary to carryout amniocentesis to look for associated chromosome abnormality (two out of twelve cases); ultrasound monitoring of a pregnancy in the woman who has previously delivered a microcephalic infant in order to diagnose a possible recurrence of the condition (three out of the twelve cases). Furthermore the authors point out tha there are limits to the method. Ultrasound is only a complementary examination and the results obtained should be studied critically, confirmed by repeated examinations and integrate into the whole clinical picture. Finally, it is not always possible to differentiate between the normal and the abnormal by ultrasound. In borderline cases ultrasound has the advantage that it can point out the need for extra careful post-natal examination of the infant.