DETERMINATION OF CHORIONICITY IN TWIN GESTATIONS BY HIGH-FREQUENCY ABDOMINAL ULTRASONOGRAPHY - COUNTING THE LAYERS OF THE DIVIDING MEMBRANE

OBJECTIVE: Our aim was to determine whether chorionicity in twin gesta tions can be diagnosed by use of high-frequency ultrasonography to cou nt the layers of intraamniotic membrane. STUDY DESIGN: This prospectiv e study of 66 twin pregnancies between 13 and 38 weeks' gestation used transabdominal ultrasonography at 10 MHz. The pregnancy was classifie d as monochorionic when two layers were counted and as dichorionic whe n 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: Ultrason ography allowed chorionicity to be determined correctly in 60 of 63 ca ses (95%; 100% in the second trimester and 92% in the third). The pred ictive value for dichorionicity was 100% (48/48) and the sensitivity 9 4% (48/51). The 12 monochorionic diamniotic pregnancies in which the m embrane was visualized were all correctly diagnosed. In a thirteenth c ase, 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 varia bility 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 b e attributable to the use of high-frequency ultrasonography.