[Comparative changes in uterine artery blood flow waveforms in singleton and twin pregnancies]

(1) To compare singleton and twin pregnancies with regard to changes in the Pulsatility Index (PI) of uterine arteries over the course of the pregnancy; (2) To examine the effect of the location of the placenta on the PI in the uterine arteries in twin pregnancies.

We prospectively studied the PI of uterine arteries in 99 women with uncomplicated singleton pregnancies and 24 women with uncomplicated twin pregnancies and examine the location of the placenta.

In both the ipsilateral and contralateral uterine arteries, the PI exhibited a steady decrease toward term in singleton pregnancies. In twin pregnancies, the PI exhibited a steady decrease until 27 weeks of gestation and remained unchanged thereafter. The PI in the ipsilateral artery was consistently lower than that in the contralateral artery in singleton as well as twin pregnancies. The mean values of PI of the ipsilateral and contralateral uterine arteries of twin pregnancies was consistently lower than that of singleton pregnancies at any gestational week.

The ipsilateral uterine artery contributes more to placental perfusion than does the contralateral uterine artery in twin as well as in singleton pregnancies. Twin pregnancies differed from singleton pregnancies with respect to changes in PI in the uterine artery according to gestational age. The PI of uterine arteries declined more rapidly and reached a plateau earlier in twin than in singleton pregnancies.