The mean peak systolic to end-diastolic (S/D) umbilical artery ratio was measured in 291 Doppler studies performed during pregnancy in 35 insulin-dependent diabetic women. A normal decline was observed in the umbilical artery S/D ratio, from 4.2 +/- 0.21 at 18 weeks to 2.18 +/- 0.22 at 38 weeks. There was no significant correlation between mean third-trimester S/D and either glycosylated hemoglobin (r = 0.25) or mean blood glucose levels (r = 0.15). Fetuses of women with vascular disease (class F/R or chronic hypertension) had a mean third-trimester S/D of 3.0 or higher in five of ten cases, compared with three of 25 in patients with uncomplicated diabetes (P less than .03). Mean second- and third-trimester S/D ratios differed significantly in patients with and without vascular disease: 4.34 +/- 0.7 and 3.2 +/- 0.65 versus 3.72 +/- 0.42 and 2.55 +/- 0.32, respectively (P less than .03). Two of three women without vascular disease who demonstrated an elevated mean S/D ratio developed preeclampsia and delivered appropriate for gestational age infants. In women with vascular disease, four of five with an abnormal mean third-trimester umbilical artery S/D ratio were delivered of growth-retarded infants, whereas all five with normal umbilical artery S/D ratios had appropriate for gestational age infants. In three of the abnormal cases, elevated S/D ratios were present in the second trimester before ultrasound documentation of fetal growth retardation.(ABSTRACT TRUNCATED AT 250 WORDS)