Prospective determination of chorionicity, amnionicity, and zygosity in twin gestations

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.