AbstractObjectiveOur purpose was to evaluate the antenatal incidence of single umbilical artery (SUA) in twin pregnancies according to chorionicity and to assess its relationship with outcome.MethodsConsecutive twin pregnancies undergoing ultrasound evaluation at our institutions were included. A targeted sonographic evaluation of the umbilical cord and vessels was performed in all cases. Chorionicity was determined according to standard ultrasound criteria.ResultsA total of 174 twin pregnancies, 100 dichorionic (DC) and 74 monochorionic (MC), were included in the study. An SUA was identified in 17 (9.8%) pregnancies, and in 18 (5.2%) fetuses. No difference was found in the incidence of SUA in DC and MC twins. Among affected pregnancies, all but one DC twin pregnancy were discordant for SUA. Structural and/or chromosomal abnormalities were present in 27.8% of fetuses with SUA. The prevalence of small‐for‐gestational‐age fetuses and of discordant birth weight (> 20% discordance) was higher in the SUA group than in the rest of the population, although these differences were not statistically significant. Twin pairs discordant for SUA had significantly higher weight discordance than those with normal umbilical cords. The sonographic cross‐sectional area of the SUA did not appear to show the typical adaptive dilatation usually seen in singleton pregnancies with SUA.ConclusionsThe incidence of SUA in twins is higher than in singletons, with no difference between MC and DC twins. Intrapair discordance for SUA in identical twins provides evidence against an exclusively genetic origin of this anomaly. The apparent failure of compensatory dilatation of the umbilical artery in twins with SUA may explain in part the higher risk for fetal growth restriction in these cases. Copyright © 2012 ISUOG. Published by John Wiley & Sons, Ltd.