ABSTRACT
Erythropoiesis (and thus Hb production) occurs in the embryonic yolk sac, then subsequently in the fetal liver and spleen. Bone marrow does not normally establish full erythropoietic potential until the second half of pregnancy, when changeover to the adult type of hemoglobin begins. This changeover continues on into neonatal life, when HbA quickly becomes predominant (Part B). In sheep, HbF has been detected up to 35 days of age, but this is thought to be a residue from that made before birth. Fetal hemoglobin in calves has been reported to account for 41% to 100% of total hemoglobin at birth, yet diminishes rapidly as it is replaced with HbA at 2 to 3 months of age. By the end of the first year, HbF is typically only 1% of total hemoglobin. On the other hand, in pigs and horses, the hemoglobin of the fetus is indistinguishable from that of the adult, and replacement of fetal blood by adult blood in humans does not seem to compromise survival of the fetus. Thus, HbF may be more of a physiologic luxury than a necessity in mammals.
