Umbilical cord structure shapes feto-maternal heat exchange across mammals

The umbilical cord exhibits striking structural diversity across placental mammals; however, the consequences of such variation on physiological function are poorly understood. Combining comparative anatomy across 130 mammal species with ancestral-trait reconstruction, multimodal imaging, and physics-based transport modelling, we show that the human umbilical cord structure is uncommon in other mammals and that the ancestral umbilical cord was unspiralled, with three vessels and an allantoic duct. Across species, vessel number and cord length correlate with birth weight: four-vessel cords and the persistence of an allantoic duct occur in species with heavier neonates, while two-vessel cords are confined to small muroid rodents. Mathematical modelling predicts that oxygen delivery remains near maximal across all cord architectures, but placental heat removal depends strongly on umbilical flow, and moderately on cord length and coiling. Scaling of umbilical flow with birth weight implies that larger species approach near-maximal heat-removal capacity, while smaller species are less efficient. Finally, three-dimensional imaging of the human cord reveals a triple-helix configuration of umbilical vessels, rather than arteries simply twisting around a central vein. This, and other observed cross-sectional vessel configurations in mammals and atypical human cords, aligns with theoretically predicted arrangements that minimize inter-vessel shunting, supporting an adaptive role for cord geometry in thermal regulation function. Together, these findings reframe the umbilical cord as a heat-exchange system and link its structural diversity to evolutionary history, neonatal mass, and maternal investment, with implications for fetal resilience to environmental stress.