Objective: To examine birth weight in pregnancies with isolated single umbilical artery (ISUA). Methods: Case control study with retrospective review of 131 singleton pregnancies with isolated single umbilical artery diagnosed before birth. Control group consisted of 730 singleton pregnancies recruited prospectively, that had histological confirmation of a 3 vessels cord. Pregnancies were classified as uncomplicated or high-risk according to the presence of diseases that increase the risk of placental insufficiency during pregnancy. Mean birth weight and frequency of low birth weight (< 2.500g), very low birth weight (< 1.500g) and fetal growth restriction below the 5th and 10th centiles were compared between groups. Results: Mean birth weight difference between ISUA (n=131, 2840 +/- 701g) and control (n=730, 2.983 +/- 671g) pregnancies was 143g (95% CI= 17-269; p=0.04) and birth weight below the 5th centile was significantly more common in ISUA group [28/131 (21.4%) versus 99/730 (13.6%), p=0.02]. When only uncomplicated pregnancies were considered in both groups, no birth weight differences were observed. Amongst high-risk subgroups, birth weight below the 5th centile remained significantly more common in ISUA compared to control pregnancies [10/35 (28.6%) versus 53/377 (14.1%), p=0.04]. Conclusion: Isolated single umbilical artery does not increase the risk of fetal growth restriction in uncomplicated singleton pregnancies.