Determination Of Chorionicity In Twin Pregnancies By Three-dimensional (3D) Ultrasonography: Measuring The Thickness Of The Inter-twin Membrane

Objective:to explore whether chorionicity can be diagnosed during the second and third trimesters by measuring the thickness of the inter-twin membrane with three-dimensional (3D) ultrasonography, and to evaluate the accuracy and the reproducibility of the technique.Methods:A prospective study was conducted in our hospital in 113 twin pregnancies whose chorionicities were assessed between 17 and 36 weeks of gestation prenatally and confirmed postnatally. The dividing membranes between the fetuses were visualized and measured with 2D and 3D transabdominal sonography respectively. We draw the receiver operator characteristic(ROC) curve to determine the optimal cut-off point of the thickness of the membranes to predict chorionicity. Intraclass correlation coefficients and Kappa index were calculated to evaluate the inter-and intraoperator variability. We also compared the accuracy of prenatal assessment of chorionicity by 2D and 3D ultrasound measurement of membrane thickness during the second and third trimesters.Results:(1) In monochorionic and dichorionic pregnancies, respectively, the mean thickness of the dividing membranes was 1.56 (SD,0.32) mm and 2.52 (SD,0.44) mm. (2) The best cut-off for membrane thickness for discriminating monochorionic from dichorionic twinning was 1.8 mm. Using this cut-off, there were two false-negative and four false-positive diagnoses of monochorionic twins with 3D ultrasound and seven false-negative and nine false-positive with 2D ultrasound. (3) With 3D ultrasound, the intraclass correlation coefficients and Kappa indices for variability between intra-and interoperator was 0.98 and 0.96 respectively. (4) When measuring the thickness of the membranes in twins using 3D ultrasound, there is a high degree of accuracy in the determination of chorionicity. The area under the ROC curve, sensitivity and specificity, were 0.906,95.4% and 94.3% respectively. In contrast,measurements with 2D sonography were less accurate. The area under the ROC curve, sensitivity and specificity, were 0.811,83.7% and 87.1% respectively.Conclusions:It is feasible,accurate and reproducible to assess chorionicity by measuring the intra-amniotic membrane thickness with 3D ultrasound, which may be a useful alternative method for combined determining chorionicity in twin pregnancies during the second and third trimesters.