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Transfer of oxygen from the mother to the fetus in human embryos. Adult and fetal hemoglobin molecules differ in their protein subunits. The fetal γ-chain binds diphosphoglycerate less avidly than does the adult β-chain. Consequently, fetal hemoglobin can bind oxygen more efficiently than can adult hemoglobin. In the placenta, there is a net flow (arrow) of oxygen from the mother's blood (which gives up oxygen to the tissues at the lower oxygen pressure) to the fetal blood, which is still picking it up.