Antenatal ultrasound diagnosis of isolated microcephaly is difficult. Over and above this, other malformations must be safely excluded. Infants with congenital microcephaly are mentally and motorially severely retarded. Basing on two case reports with four microcephalic foetuses, the antenatal problems of sonographic diagnosis are explained. In all cases there was an isolated microcephaly without other malformations. Sonographic examination enabled early detection of three microcephalic foetuses between the 15th and 32nd week of gestation, and pregnancy could be terminated prematurely. Spontaneous delivery occurred in one pregnant woman at term; sonography had not been employed in this case.