The objective of this study was to identify abnormal vascular coiling of the umbilical cord in neonates of mothers with gestational diabetes mellitus. The umbilical cords of 57 neonates of gestational diabetic mothers were examined and the coiling index determined by dividing the total number of complete vascular coils by the length of the cord in centimeters. Obstetric history, delivery data and neonatal outcome were also evaluated. These variables were compared with those obtained for 389 normal pregnancies. The frequency distribution of umbilical coiling index in the control population and gestational diabetic mothers were normal (10th and 90th percentiles = 0.17 and 0.37; mean +/- SD = 0.26 +/- 0.09 and 0.24 +/- 0.12 coils/cm, respectively). Non-coiling and hyper-coiling were significantly more frequent with diabetic than with normal pregnancy (p = 0.004; p = 0.008, respectively). Both abnormalities of umbilical vascular coiling were also significantly associated with adverse perinatal outcome (p = 0.04) and emergency cesarean delivery (p < 0.0001) in the diabetic and control (p = 0.03; p < 0.0001, respectively) groups. Neonates of gestational diabetic mothers are therefore more likely to have hyper-coiled or non-coiled umbilical blood vessels. Perinatal morbidity and emergency cesarean delivery are increased in this subgroup.