OBJECTIVE: Our purpose was to measure umbilical blood flow continuously by use of a transit time ultrasonic flow transducer and to compare the blood flow measurements with the steady-state diffusion method in the chronic fetal sheep preparation. STUDY DESIGN: We compared umbilical blood flow measurements calculated by the steady-state diffusion method with ethanol as the diffusing substance and with the transit time ultrasonic flow transducer placed on the common umbilical artery in five chronically prepared fetal sheep. RESULTS: There was no statistical difference between measurements of umbilical blood flow measured by the flow transducer versus the steady-state diffusion method, 600 ± 22 versus 664 ± 56 ml per minute (mean ± SEM) ( p = 0.23). The mean coefficient of variation within each study was 13.6% for the steady-state diffusion method versus 4.1% for the transit time flow transducer. Umbilical blood flow variance was significantly lower as measured by the flow transducer compared with the diffusion method ( p < 0.0001). There were no differences in umbilical blood flow per kilogram or fetal oxygen uptake between the two methods. CONCLUSION: We conclude that umbilical blood flow can be measured continuously under steady-state conditions by use of a transit time flow transducer. Because of the lower variability in the flow transducer–obtained measurements, we speculate that the flow transducer may differentiate alterations in umbilical blood flow with greater precision in chronic preparations. This may be advantageous for measuring absolute changes in fetal substrate uptake, especially under non-steady-state conditions. (AM J OBSTET GYNECOL 1996;174:1456-60.)