The purpose of this study was to investigate vascular resistance by Doppler ultrasound in the umbilical artery of insulin-dependent diabetics longitudinally over the course of pregnancy. Special interest was put on the effects of glycemic control and maternal vascular disease on the flow velocity waveform (FVW) and the predictive value of Doppler flow measurements with regard to perinatal morbidity. Using a duplex-pulsed wave scanner, the resistance-index (PR index) in the umbilical artery was calculated. The mean value of a 24-h blood glucose profile and the concentration of glycosylated hemoglobin (HBA1C) were used as parameters of metabolic control. 53 pregnant diabetic women were examined longitudinally during the course of pregnancy on average on three occasions (range: 1-7) between 17 weeks of gestation and delivery at 37.7 +/- 1.3 (mean +/- SD) weeks. To test the predictive value of Doppler flow velocimetry with regard to perinatal morbidity the results were compared to the FVWs measured in the umbilical arteries of 30 non-diabetic women with normal fetal outcome. Vascular resistance in the umbilical artery of the diabetics declined significantly during the course of pregnancy, with a mean PR index of 0.729 (SD 0.051) at 17 weeks and 0.603 (SD 0.083) at the end of pregnancy (P < 0.002). The majority of PR indices were within the range reported for normal pregnancy and measured in the non-diabetic women. Regression analysis showed no significant correlation between vascular resistance and mean blood glucose level (r = 0.1325) or concentration of HBA1C (r = -0.0519). Maternal vascular disease had no effect on umbilical FVWs.(ABSTRACT TRUNCATED AT 250 WORDS)