Color Doppler sonography was used to study umbilical and ductus venosus (DV) flow in 137 normal fetuses between 20 and 38 wk of gestation. Hepatic flows were also evaluated. In all parts of the venous circulation examined, blood flow increased significantly with advancing gestational age. The weight-specific amniotic umbilical flow did not change significantly during gestation (120 +/- 44 ml. min(-1). kg(-1)), whereas DV flow decreased significantly (from 60 to 17 ml. min(-1). kg(-1)). The percentage of umbilical blood flow shunted through the DV decreased significantly (from 40% to 15%); consequently, the percentage of flow to the liver increased. The right lobe flow changed from 20 to 45%, whereas the left lobe flow was approximately constant (40%). These changes are related to different patterns of growth of the umbilical veins and DV diameters. The present data support the hypothesis that the DV plays a less important role in shunting well-oxygenated blood to the brain and myocardium in late normal pregnancy than in early gestation, which leads to increased fetal liver perfusion.