OBJECTIVE: Our purpose was to determine the predictive accuracy of a composite ultrasonographic evaluation for chorionicity, amnionicity, and zygosity in a consecutive series of twins.STUDY DESIGN: One hundred ten consecutive twins were seen for ultrasonography beginning January 1992. Chorionicity, amnionicity, and, zygosity were prospectively assessed with a composite of ultrasonographic findings (placental number, fetal sex, membrane thickness, and ''twin peak'' sign). Clinical and pathologic confirmation of chorionicity, amnionicity, and zygosity was available on 100 of these twins.RESULTS: The 100 twins had 3.6 +/- 1.6 ultrasonographic scans each (mean +/- SD) with the first performed at 22.6 +/- 6.9 weeks. Chorionicity, amnionicity, and zygosity were each predicted with >=91% sensitivity and specificity. In 35 (35%) cases zygosity could not be determined by either ultrasonographic or clinical or pathologic assessmen at delivery.CONCLUSION: Chorionicity, amnionicity, and zygosity have important implications for antepartum management and prognosis of twins. By use of a composite of ultrasonographic findings, chorionicity, amnionicity, and zygosity were predicted with excellent reliability when they were prospectively tested in a heterogeneous consecutive series of twins.