The flow pattern in intrarenal veins depends on renal parenchymal histology and cardiac physiology. The intrarenal venous impedance index obtained by Doppler ultrasound is related to compliance in vein, and can be helpful in the assessment of renal parenchymal compliance. The purpose of this study was to determine whether normal pregnancy has a significant effect on intrarenal venous blood flow, and assess if the physiological pyelocaliectasis causes a measurable reduction in venous impedance indexes in pregnant women. Doppler ultrasound of intrarenal veins was performed in 35 asymptomatic pregnant women in the second and third trimester of gestation, and in 24 non-pregnant healthy women. After grading the degree of hydronephrosis, venous impedance index was obtained from the interlobar veins. The venous waveforms in pregnant women showed diminished phasic oscillations owing to elevated pre-systolic flow. The mean venous impedance indexes in pregnant women were significantly lower than the values in non-pregnant subjects, 0.30+/-0.10 versus 0.44+/-0.06 in the right (p<0.001), and 0.36+/-0.11 versus 0.41+/-0.07 in the left kidney (p=0.03). There was an inverse correlation between the grade of pelvicalyceal dilatation and the venous impedance indexes in both kidneys in pregnant women (r=-0.62, p<0.001 for the right kidney, and r=-0.38, p=0.05 for the left kidney). An abnormally reduced venous impedance index in pregnant women can at least in part be explained by reduced vascular compliance from increased interstitial pressure subsequent to partial obstruction of ureters by the gravid uterus, and caution should be exercised in interpreting it as a sign of pathological ureteral obstruction.