Influence of parity on second-trimester uterine artery Doppler waveforms in twin pregnancy.

We performed uterine artery Doppler velocimetry in 16 nulliparous and 16 parous women with non-preeclamptic singleton gestations and 16 nulliparous and 16 parous women with dichorionic twin gestations at 17-18 and 26-27 weeks of gestation. In both singleton and twin pregnancies, the average pulsatility index (PI) in nulliparous women was significantly higher than that in parous women at 17-18 weeks of gestation. At 26-27 weeks of gestation, however, there were no significant differences in PI between nulliparous and parous women with either singleton or twin pregnancies. During the early second-trimester of non-preeclamptic singleton and twin pregnancies, parity has a significant effect on uterine artery blood flow.