Umbilical cord structure shapes feto-maternal heat transfer across mammals

The umbilical cord varies widely across placental mammals, yet the functional consequences of this structural diversity remain unclear. We compared cord anatomy across 130 mammal species, reconstructed ancestral states, and combined multimodal imaging with physics-based modelling of oxygen and heat transfer. Reconstructions indicate that the ancestral umbilical cord was uncoiled, with three vessels and an allantoic duct; the human combination of coiling with loss of the duct is rare. Across species, vessel number, duct persistence, and cord length increase with birth weight. Modelling predicts that oxygen delivery remains near maximal across architectures, but that placental heat loss depends more strongly on umbilical flow and on cord length and coiling, suggesting more efficient cooling in larger mammals. Microscopy and ultrasound imaging reveal geometric vessel layouts, including a human triple helix, consistent with configurations predicted to reduce inter-vessel heat shunting. As fetuses must dissipate heat through the mother, these results link cord morphology, evolution, and fetal thermal resilience.