Transvaginal color Doppler was used to investigate the uteroplacental circulation of 45 patients with normal intrauterine pregnancies at 4-18 weeks' gestation. The main uterine artery and the radial and spiral arteries were demonstrated, and characteristic flow velocity waveforms were obtained in more than 90% of cases. The indices of impedance to flow decreased with gestation and there was a progressive fall in these indices from the uterine artery, through the radial, to the spiral artery. Blood velocity in the uterine artery increased exponentially with gestation.