Fetal erythrocytes are produced in different locations throughout the life of the fetus.
- Yolk sac (3–8 weeks) during organogenesis
- Liver (7 weeks–birth)
- Spleen (9–28 weeks)
- Bone marrow (22 weeks–adult axial skeleton [pelvis, ribs, sternum, vertebrae] and long bones’ proximal epiphyses)
Fetal hemoglobin consists of two alpha subunits and two gamma subunits (α2 and γ2).
Because fetal hemoglobin has a higher affinity for oxygen due to its lower affinity for 2,3-bisphosphoglycerate (2,3-BPG) than does adult hemoglobin, the transfer of oxygen across the placenta from maternal to fetal circulation is ensured. After birth, there is a gradual decrease in red cell production, caused by increased oxygenation of systemic circulation, and a switch from fetal to adult hemoglobin (consists of two alpha and two beta subunits). This results in a physiologic anemia that nadirs around 4–8 weeks of life before a new steady-state production of adult hemoglobin is established.