Reference ranges for the pulsatility index of the fetal aortic isthmus in singleton and twin pregnancies

The purpose of this study was to estimate reference ranges for the pulsatility index (PI) of the fetal aortic isthmus in uncomplicated singleton and twin pregnancies during the second half of pregnancy.

We conducted a cross-sectional observational study involving 543 healthy fetuses: 361 singleton pregnancies and 182 twin pregnancies between 19 and 36 weeks' gestation. The aortic isthmus PI was measured in 2 sonographic planes: the longitudinal aortic arch view and the 3-vessel and trachea view. We evaluated the reproducibility of aortic isthmus PI measurements between these planes by calculating intraclass correlation coefficients and limits of agreement. Scans were performed by 2 physicians, and intraobserver agreement was also measured. Regression analysis was used to estimate gestational age reference values for the aortic isthmus PI.

The aortic isthmus PI was significantly correlated with gestational age in singletons and twins during the second half of pregnancy (P < .01). We did not find significant differences between reference ranges in singletons and twins or between the sonographic views.

This study offers reference ranges for the aortic isthmus PI during the second half of gestation in singleton and twin pregnancies. Mean aortic isthmus PI values were similar in both types of gestations as well as both sonographic views. The aortic isthmus PI may be reliably obtained from either sonographic view.