OBJECTIVE. We determined the accuracy of prenatal sonography for detecting placental circumvallation, a placental abnormality associated with increased fetal morbidity and mortality. MATERIALS AND METHODS. We analyzed 62 healthy pregnant (range, 18-36 weeks) patients with focused placental sonography for detection of morphologic abnormality using the published criteria for circumvallate placenta (irregular edge, uplifted margin, or placental sheet or shelf). Placental marginal sonograms were taken at 30°intervals around the entire placental margin. Five experienced sonologists who were unaware of the pathologic findings independently reviewed the placental images and graded the placentas from 1 (definitely normal) to 5 (definitely circumvallate). Receiver operating characteristic (ROC) curves and area under the ROC curve were calculated for each reader. Gross and microscopic pathology was used as the gold standard for all cases. RESULTS. In the 62 patients, sonography revealed 49 normal placentas (79%) 12 partially circumvallate placentas (19%), and one completely circumvallate placenta (2%). ROC curves for the reviewers resulted in values for area under the curve ranging from .39 to .58. The sonologist who achieved the highest value for the area under the curve classified the 13 cases of proven circumvallation as one case of definite circumvallation, four cases as uncertain or equivocal, and eight cases as probably or definitely normal placentas. Of the normal placentas, 35% were graded as probably or definitely circumvallate by at least one sonologist. All sonologists misgraded the case of complete circumvallation as normal. CONCLUSION. Focused placental sonograms interpreted by experienced sonologists fail to detect the placental edge abnormality in most cases of circumvallation. In our study, 17 of 49 normal placentas were diagnosed as probably or definitely circumvallate by one or more observers. Our sonologists' interpretations of sonograms showed that complete circumvallation was difficult to assess. Although our study had a limited number of patients, the accuracy of sonography of the placenta for revealing circumvallation appears to be limited.