Recent data suggest that umbilical venous perfusion of the fetal liver has an important influence on fetal growth and postnatal liver function, and that maternal factors in late pregnancy modify this circulation. In a longitudinal study of 160 low-risk pregnancies, we determined how umbilical and portal venous blood flows to the fetal liver changed during gestation, and examined the hypothesis that maternal body mass index and pregnancy weight gain influenced fetal liver blood flows. We measured blood flows in the umbilical and portal veins, left portal branch, and ductus venosus using ultrasound. Normalizing for estimated fetal weight, fetal liver total venous blood flow fell from 84 to 57 mL. min(-1). kg(-1) during 21-39 wk of gestation; toward term the portal contribution increased (from 14 to 20%) and the umbilical contribution fell, whereas distribution between the left and right liver lobes was stable, 60%/40%. Greater flow of nutrient-rich umbilical venous blood to the liver was associated with higher birth weight and neonatal ponderal index. Maternal body mass index was not related to fetal liver blood flows, but low pregnancy weight gain strongly influenced flow distribution between the right and left liver lobes, sparing the left lobe and increasing the difference between lobes by 16%.