OBJECTIVES To compare uterine artery pulsatility index (PI) obtained at 11 + 0 to 13 + 6 weeks of gestation in singleton and twin pregnancies and to evaluate changes in PI values of twin pregnancies developing pre-eclampsia (PE) or small-for-gestational age (SGA) of either one or both fetuses. METHODS Uterine artery PI was measured in 421 twin pregnancies (384 dichorionic and 37 monochorionic) and in 500 singleton pregnancies. The measured mean and lowest uterine artery PI values were converted to multiples of the expected normal median (MoM) after correction for maternal body mass index, ethnicity and gestational age. The median PI-MoM values of twins were compared with those of singleton pregnancies. In twin pregnancies, PI-MoM values were analyzed according to chorionicity, development of early-onset (< 34 weeks) or late-onset (≥ 34 weeks) PE and SGA of one or both twins. RESULTS Uterine artery PI-MoM was significantly lower in twin compared with singleton pregnancies (mean K = 174.31, P < 0.0001, lowest K = 139.27, P < 0.0001). However, there were no significant differences in the uterine artery PI-MoM values between monochorionic and dichorionic twins. The uterine artery PI in twin pregnancies that developed early-onset PE (P < 0.001) and SGA of both twins (P < 0.05) was higher than the uterine artery PI in uncomplicated twin pregnancies, whereas no differences were found for late PE or SGA of one twin. CONCLUSIONS First-trimester placental impedance to flow, as assessed by uterine artery Doppler examination, is reduced in twin pregnancies, with no differences related to chorionicity. The relative increase of uterine artery PI found in twin pregnancies that developed early PE and SGA of both twins suggests that first-trimester uterine artery assessment may be useful in identifying such complications.