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We thank Dr Napolitano and Professor Thilaganathan for their interest in our recent article.1Klein K. Mailath-Pokorny M. Elhenicky M. Schmid M. Zeisler H. Worda C. Mean, lowest, and highest pulsatility index of the uterine artery and adverse pregnancy outcome in twin pregnancies.Am J Obstet Gynecol. 2011; 205: 549.e1-549.e7Abstract Full Text Full Text PDF Scopus (20) Google Scholar In their letter to the editors, they were concerned about the interpretation of our results according to the use of highest pulsatility index (PI) of the uterine arteries, measured at 20-22 weeks of gestation, for risk assessment of preeclampsia in twin pregnancies. They stated that we would favor the use of highest PI for risk assessment of preeclampsia, although no significant difference was found when using the mean or lowest PI. From the clinical point of view, it makes sense to use the PI, which represents the highest relative risk for preeclampsia in twin pregnancies, and this is in our study, the highest PI. Although we have observed a wider range of relative risk confidence interval for preeclampsia even the 2.5th percentile increases the risk more than 6-fold. Therefore, it demonstrates a robust increase in risk from the statistical point of view. In general, as long as specific questions are not evaluated in populations of twin pregnancies, findings of studies on singleton pregnancies need to be extrapolated to twin pregnancies but should be interpreted carefully. Mean, lowest, and highest pulsatility index of the uterine artery and adverse pregnancy outcome in twin pregnanciesAmerican Journal of Obstetrics & GynecologyVol. 206Issue 6PreviewWe read with great interest the article published by Klein et al1 on uterine artery Doppler screening for preeclampsia and adverse twin pregnancy outcome in the December issue of the Journal. The authors confirmed that the reference range for uterine artery Doppler pulsatility indices (PIs) are different in twins compared with singleton pregnancy and that there is a significant difference in PI between twin and singleton pregnancy. Finally, they demonstrated quite convincingly a significant association between increased PI and adverse pregnancy outcome, presumably as a consequence of impaired placental development. Full-Text PDF