The small size of the head and abdomen in a growth-retarded fetus may lead to inaccurate estimates of gestational age. Therefore, we conducted a study to assess the diameters of the aortic root and pulmonary artery in such fetuses. During the study period, we measured these great vessel diameters at the level of the semilunar valves on all third-trimester obstetric sonograms in which intrauterine growth retardation was suspected. Cases were included in the study if delivery occurred within 1 week of the scan and if growth retardation was confirmed after birth. The study population consisted of 75 growth-retarded fetuses. We compared the relationships between great vessel diameters and gestational age in this study population with norms previously established in our laboratory in 403 normal fetuses. In a similar fashion, we compared the relationships between great vessel diameters and biparietal diameter in our study population with previously established norms. We found no significant difference between great vessel diameters in growth-retarded fetuses and those in normal fetuses of the same gestational age (p greater than .45 for aorta; p greater than .40 for pulmonary artery). In contrast, aortic root diameter was larger in relation to biparietal diameter in growth-retarded fetuses than it was in normal fetuses (p less than .05), and there was a tendency toward the same result for the pulmonary artery. We conclude that the diameters of the aorta and pulmonary artery remain normal in most cases of intrauterine growth retardation. The aorta and pulmonary artery may be useful predictors of gestational age when growth retardation is suspected.