This article has detailed the common prenatal sonographic findings that have been detected in fetuses with Down syndrome, trisomy 13, trisomy 18, Turner syndrome, and triploidy. It should be noted that not all fetuses with these five chromosomal abnormalities will have sonographically detectable malformations and that some may not be easily recognizable even at birth. It is crucial that the approach to the patient with a sonographically detected fetal malformation be thorough, systematic, and expeditious. The first step is to attempt to accurately define the abnormality. Next, syndromes with which this malformation are associated should be researched and a differential diagnosis constructed. Based on this differential, accompanying findings with which this abnormality is often associated should be looked for sonographically. A detailed history including pedigree information, possible teratogen exposure, consanguinity, and ancestry is imperative. In some cases a Mendelian disorder may be identified by obtaining a thorough prenatal genetic history. In virtually all cases of prenatally detected fetal malformations, chromosome analysis is indicated. Amniocentesis is the most common means of fetal chromosome analysis, but usually takes at least 2 weeks for results. In cases where the gestation is approaching the legal limit for elective termination, rapid karyotyping by percutaneous umbilical cord sampling should be considered and referral to a center familiar with this procedure is suggested. Chromosome analysis from fetal blood takes approximately 3 days to accomplish. Alternatively, rapid karyotyping by transabdominal chorionic villus biopsy may also be accomplished and has been performed as late as 37 weeks gestation. This procedure may be especially advantageous in cases where oligohydramnios is present. If a chromosomal abnormality is identified, genetic counseling should be arranged to discuss prognosis, cause, recurrence risks and to help the parents through this difficult time. If applicable, the option of pregnancy termination should be offered. Not all parents faced with a chromosomally abnormal fetus will wish to terminate their pregnancy and others may be too late in gestation to consider this option. In these cases, discussions with other families raising similarly affected children, social workers, clergy, psychiatrists, and geneticists may be helpful to the parents. Late pregnancy prenatal cytogenetic diagnosis may be beneficial both to the parents in helping prepare them psychologically and emotionally for an adverse outcome and in guiding obstetric management.(ABSTRACT TRUNCATED AT 400 WORDS)