OBJECTIVE: Our purpose was to establish reference data representative of normal findings at transvaginal color and spectral Doppler ultrasonographic examination of the uteroplacental and luteal circulation in the first trimester and to relate the Doppler findings to morphologic data. STUDY DESIGN: A cross-sectional study was performed of 64 uncomplicated pregnancies of 5 to 11 completed gestational weeks terminated by legal abortion for psychosocial reasons. Doppler examinations of the main uterine arteries, subchorionic arteries, intrachorionic area, and corpus luteum were performed. Abortion material was examined morphologically. RESULTS: In the uterine and subchorionic arteries blood flow velocity increased and pulsatility index values decreased with advancing gestation. Arterial or venous Doppler shift spectra were recorded from the intrachorionic area in 91% (58/64) of the pregnancies. Intrachorionic arterial blood flow velocities and pulsatility index values were low and did not change with advancing gestation; the mean time-averaged maximum velocity, peak systolic velocity, and pulsatility index values were 4.6 cm/sec, 6.0 cm/sec, and 0.47, respectively. Morphologic examination showed trophoblast plugs in the spiral arteries to consistently manifest multiple spaces containing red blood cells at the choriodecidual junction. CONCLUSION: Blood flow velocity in the uteroplacental arteries increases and pulsatility index values decrease with advancing gestation, indicating a steady increase in uterine perfusion during the first trimester. Our findings suggest intervillous flow to be present as early as the first trimester