Because beta globin is naturally low at birth, beta-thalassemia and structural abnormalities of the beta globin chain (eg, Hb S [sickle cell disease], Hb C) are not clinically apparent at birth and symptoms do not appear until fetal hemoglobin levels have fallen to sufficiently low levels at 3 to 4 months of age and are replaced by adult hemoglobin containing either a pathologic mutation in the beta chain (as in sickle cell anemia) or a decreased percentage of beta chain (as in beta-thalassemia).