Unsuspected fetal infection (viral or parasitic) is often overlooked as a possible cause of abnormalities revealed on ultrasonographic examination during pregnancy. Infectious agents can produce a wide spectrum of ultrasonographic findings. Our objective was to highlight those findings and review the rationale and method of antenatal investigation. No ultrasonographic findings are pathognomonic for a particular agent. The search for possible fetal infection consists of both direct and indirect techniques. Methods to directly demonstrate fetal infection include maternal-fetal serologic studies, culture, electron microscopy, and polymerase chain reaction. Emerging evidence strongly suggests that prior assumptions concerning the sensitivity of both maternal and fetal-neonatal serologic studies are incorrect. Serologic studies alone are inadequate to exclude fetal infection as a cause of an ultrasonographically detected abnormality. Indirect techniques suggestive of fetal infection include fetal hematologic and biochemical measurements. New approaches and methods for the detection of fetal infection are needed. The evaluation should be initiated antenatally. Needless delay until after delivery increases the likelihood that the diagnosis will be missed. Antenatal diagnosis provides the opportunity for therapy and often leads the practitioner to modify the obstetric and neonatal care plan. A protocol used by the University of Iowa Fetal Diagnosis and Treatment Unit for the diagnosis and management of unsuspected fetal infection is offered as an example of a protocol that has been successful in the past.