Fetal erythropoiesis (Young Liver Synthesizes Blood)
Yolk sac (3–8 weeks)
Liver (6 weeks–birth)
Spleen (10–28 weeks)
Bone marrow (18 weeks to adult)
Hemoglobin development (From fetal to adult hemoglobin:α Always; ɣ Goes, Becomes β)
Embryonic globins: ζ and ε.
Fetal hemoglobin (HbF) = α2γ2.
Adult hemoglobin (HbA1) = α2β2.
HbF has higher af nity for O2 due to less avid binding of 2,3-BPG, allowing HbF to extract O2 from maternal hemoglobin (HbA1 and HbA2) across the placenta. HbA2 (α2δ2) is a form of adult hemoglobin present in small amounts.