PROSPECTIVE DETERMINATION OF CHORIONICITY, AMNIONICITY, AND ZYGOSITY IN TWIN GESTATIONS

OBJECTIVE: Our purpose was to determine the predictive accuracy of a c omposite ultrasonographic evaluation for chorionicity, amnionicity, an d 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 ass essed 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 .5 ultrasonographic scans each (mean +/- SD) with the first performed at 22.6 +/- 6.9 weeks. Chorionicity, amnionicity, and zygosity were ea ch predicted with greater than or equal to 91% sensitivity and specifi city. In 35 (35%) cases zygosity could not be determined by either ult rasonographic or clinical or pathologic assessment at delivery. CONCLU SION: Chorionicity, amnionicity, and zygosity have important implicati ons for antepartum management and prognosis of twins. By use of a comp osite of ultrasonographic findings, chorionicity, amnionicity, and zyg osity were predicted with excellent reliability when they were prospec tively tested in a heterogeneous consecutive series of twins.