OBJECTIVE: The objective of this study was to elucidate the embryologic basis of fused umbilical arteries in the human. STUDY DESIGN: Twenty-nine human embryo specimens at Carnegie stages 11 through 15 (4-5 weeks after fertilization) were examined histologically, with special reference to the development of umbilical arteries. RESULTS: All embryos at Carnegie stage 11 and 12 had fused umbilical arteries, and 66% of Carnegie stage 13 embryos and 29% of Carnegie stage 14 embryos still had the condition. None of the embryos at Carnegie stage 15 or older had fused umbilical arteries, but there were always 2 arteries present in their umbilical cords. CONCLUSION: Our data suggest that (1) a single umbilical artery splits into 2 as the developmental stage of the embryo advances, (2) that fused umbilical arteries represent a remnant of the embryonic phenotype, and (3) that fused umbilical arteries are embryologically distinct from true single umbilical artery.