OBJECTIVE: Our aim was to determine whether chorionicity in twin gestations can be diagnosed by use of high-frequency ultrasonography to count the layers of intraamniotic membrane. STUDY DESIGN: This prospective study of 66 twin pregnancies between 13 and 38 weeks' gestation used transabdominal ultrasonography at 10 MHz. The pregnancy was classified as monochorionic when two layers were counted and as dichorionic when three or four layers were counted. The findings of the examiner, who had no other information about chorionicity, were compared with those of the histopathologic examination of the placenta. RESULTS: Ultrasonography allowed chorionicity to be determined correctly in 60 of 63 cases (95%; 100% in the second trimester and 92% in the third). The predictive value for dichorionicity was 100% (48/48) and the sensitivity 94% (48/51). The 12 monochorionic diamniotic pregnancies in which the membrane was visualized were all correctly diagnosed. In a thirteenth case, with severe oligohydramnios, the membrane could not be seen. Two patients were lost to follow-up. In 95% of the cases (63/66) only one examination was required to diagnose chorionicity. Intraobserver variability was 0% (0/26). Interobserver variability, tested by photographs, was 3% (2/65). CONCLUSIONS: This technique should be the first-line method for determining chorionicity in the second and third trimesters because it is the most effective. Its excellent reproducibility may be attributable to the use of high-frequency ultrasonography. (Am J Obstet Gynecol 1996;175:1529-33.)