OBJECTIVES: Our objectives were two-fold: 1) to determine the frequency of discordant umbilical artery Doppler systolic to diastolic (S/D) ratios in the individual umbilical arteries of growth-restricted fetuses and 2) to examine the impact of the frequency of discordance on clinical outcomes. METHODS: This was a prospective, observational study of growth-restricted fetuses. Doppler velocimetry was performed weekly and two S/D ratios were obtained for each fetal umbilical artery. Inter-artery discordance was defined as a difference in measurement categories (i.e., normal, elevated, absent, reversed) between the arteries. The number of abnormal measurements per visit was summed to 0-4 out of 4 values. A composite average number of abnormal Doppler measurements was calculated and fetuses were stratified based on degree of average number of abnormalities in increments of 25%: 0-<25%, 25-<50%, 50-<75%, and 75-100% abnormality. RESULTS: Of a total 241 fetuses (1762 visits), 110 (45.6%) had abnormal UAD flow and 189 (66%) demonstrated discordance. Abnormal values were noted in only one artery in 53% (n=151) of visits. Fetuses with any abnormal Doppler testing had smaller birthweights compared to fetuses with consistently normal testing (2485g vs 2623g, p <0.01); birthweight decreased as composite average of abnormal measurements increased (p = 0.03). CONCLUSION: The majority (66%) of fetuses with abnormal testing demonstrated UAD discordance. Up to 53% of fetuses could have been misdiagnosed if only one artery was tested. Fetuses with a higher frequency of Doppler abnormalities had lower birthweights. We propose obtaining two measurements from each umbilical artery in growth-restricted fetuses.