How does the interaction between BPG and fetal hemoglobin affect oxygen transport during pregnancy?

Fetal hemoglobin's low affinity for BPG increases its oxygen affinity, facilitating oxygen transfer from maternal blood.

BPG has no effect on fetal hemoglobin's oxygen affinity.

BPG increases fetal hemoglobin's oxygen affinity by binding to gamma subunits.

BPG binds more strongly to fetal hemoglobin, reducing its oxygen affinity.