In 55 women with singleton pregnancies, colour flow mapping and pulsed wave velocimetry were used to measure impedance to flow in the uterine arteries at 10–13 weeks gestation and again at 19–22 weeks. In the first trimester, examinations were performed both transabdominally and transvaginally and in the second trimester transabdominally only. There were significant associations between the first- and second-trimester measurements obtained with both Doppler techniques. These associations were higher when transvaginal than transabdominal Doppler was used and when the measurement of impedance was the pulsatility index (PI) rather than the resistance index. These data suggest that impedance to flow in the uteroplacental circulation in the second trimester is dependent on impedance in the first trimester. In any prospective, first-trimester, uterine artery Doppler screening study for pregnancy complications, it may be preferable to use transvaginal Doppler and measure