The Role of Second and Third Trimester Umbilical Artery Doppler Ultrasonography in Determining Chorionicity of Twin Pregnancies

Abstract

Introduction: Determining the chorionicity in monozygotic twins is of pivotal importance, because it has been shown that monochorionic fetuses compared to dichorionic counterparts bear a higher risk f morbidity and mortality. Using conventional ultrasonography in this regard is along with accurate results provided that it is conducted before 14 weeks’ gestation. It has been suggested that differences between color Doppler ultrasonographic indices obtained from the umbilical arteries of twins may result in the prediction of chorionicity. Nevertheless, relevant available data are very scarce. This study aimed to examine the role of umbilical artery Doppler ultrasound in the second and third trimesters in determining chorionicity in twin pregnancies Methods : A total 20 monochorionic monozygotic pregnancies and 30 dichorionic monozygotic pregnancies between 28 and 39 weeks’ gestation were examined by color Doppler study of the umbilical arteries. The pulsatility index (PI), resistive index (RI) and the S to D ratio (S/D) of the umbilical arteries and their differences in each group independently were compared between twins. Results: The mean PI difference between the two umbilical arteries did not differ significantly between the two groups; however, the mean differences of RI and S/D were significantly higher in the dichorionic group (0.006 ± 0.01 vs. 0.04 ± 0.01, P = 0.02, and 0.54 ± 0.09 vs. 0.33 ± 0.05, P = 0.04, respectively). When the difference between the two fetuses in terms of the umbilical artery RI difference exceeded 0.05 and over, the probability of dichorionic pregnancy was present (sensitivity, 60% and specificity, 65%). The difference of S/D was not acceptable as to the diagnostic accuracy in this regard. Conclusions: On the basis of our findings, the results of umbilical artery color Doppler study in twin pregnancies aged 28-39 weeks’ gestation could be used for determining chorionicity. To reach more solid conclusions, further studies with larger sample sizes are recommended to be carried out.