The ratio of peak systolic to least diastolic flow velocity (S/D) of the umbilical artery was measured in 119 pregnant women and its relation with placental pathology were studied. Results obtained indicated that S/D ratio decreased gradually with advanced gestational age and showed a negative correlation (r = -0.972 8, P < 0.01). A ratio of < 3 was defined as normal value. Patients were divided into 2 groups according to the S/D ratio and their placental histological findings were compared. The number of small arteries in the tertiary villi stem and syncytial vascular membranes were significantly less in group 2 (S/D > or = 3) than that in group 1 (S/D < 3, P < 0.01). The counts of villous syncytial knots and stroma fibrosis of villi were more significant in the placenta of group 2 as well. The mean value of molondialdehyde in placenta of group 2 was significantly higher (P < 0.01) than that of group 1, while superoxide dismutase value is to the contrary (P < 0.01). These indicated that there was a reduced circulation resistance in placenta during normal pregnancy and remained stable till term. When S/D > or = 3, it indicated that physiological and biochemical changes occurred in placenta and the outcome of pregnancy might be affected.