Based on the structural differences between fetal and adult hemoglobin, predict the direction of oxygen flow between maternal and fetal blood.

• A. Oxygen flow is bidirectional between maternal and fetal blood.
• B. Oxygen flows from fetal blood to maternal blood due to adult hemoglobin's higher oxygen affinity.
• C. Oxygen does not flow between maternal and fetal blood.
• D. Oxygen flows from maternal blood to fetal blood due to fetal hemoglobin's higher oxygen affinity.

Answer: (D)