Uterine artery Doppler velocity waveforms in twin pregnancies

Objective: To compare uterine artery resistance index values in twin and singleton pregnancies, to examine eventual modifications of these values in twin pregnancies complicated by gestational hypertension and preeclampsia, and to determine whether resistance index values in twin pregnancies could predict the development of gestational hypertension and preeclampsia. Methods: In a cross-sectional study, reference limits for gestation were constructed for the uterine artery resistance index (higher, lower, and mean values) in 96 uncomplicated twin pregnancies and compared to the reference limits constructed from 315 normal singleton pregnancies. Uterine artery resistance indexes obtained in 53 twin pregnancies complicated by either gestational hypertension or preeclampsia were compared with the newly established nomograms. The clinical efficacy of the uterine artery resistance index to predict hypertensive complications was evaluated prospectively in 64 twin pregnancies studied at 20-24 weeks' gestation. Results: In both singleton and twin pregnancies, uterine artery resistance indexes decreased linearly with advancing gestation. However, twin pregnancies showed significantly different slopes and constant values, resulting in lower resistance indexes at all gestational ages examined. No significant differences were found when comparing resistance indexes in all patients with gestational hypertension or preeclampsia to the reference limits. Statistically significant differences were obtained for the higher (P less-than-or-equal-to .05) and mean (P less-tha-or-equal-to .01) resistance indexes when the comparison was restricted to preeclamptic patients. In the twin pregnancies studied at 20-24 weeks' gestation, the diagnostic efficacy of the uterine artery resistance index for predicting the development of gestational hypertension and/or preeclampsia was disappointingly low (k < 0.10). Conclusions: Resistance index values in the uterine artery are lower in twin pregnancies than in singleton pregnancies. Gestational hypertension and preeclampsia may occur in twin pregnancies despite normal uterine artery velocity waveforms, suggesting a limited role of this measurement in the management and prediction of hypertensive complications in twin pregnancies.