This report describes a series of 5 fetuses with fused umbilical arteries that had the prenatal feature of a single umbilical artery near the placental insertion and the normal 2 umbilical arteries at the fetal end of the cord. In 1 case this vascular anomaly was associated with unilateral renal agenesis in a fetus with a subsequent diagnosis of Hallermann-Streiff syndrome. No perinatal complications were identified in the remaining 4 fetuses. Postpartum examination of the cord revealed that the 2 umbilical arteries fused to form 1 artery for a long segment of the distal portion of the cord. Our findings suggest that the prenatal evaluation of the umbilical cord to document the number of vessels should include multiple views of the cord and demonstration of the 2 intra-abdominal umbilical arteries with color Doppler imaging for a confident diagnosis. Our observation also suggests that, at least in some cases, single umbilical artery may result from incomplete splitting of the single artery normally present in early human embryos.