Complex sentence: In newborns, fetal hemoglobin is nearly completely replaced by adult hemoglobin by approximately 6 months postnatally, except in a few thalassemia cases in which there may be a delay in cessation of HbF production until 3-5 years of age.
Split into simpler sentences: In newborns, fetal hemoglobin is nearly completely replaced by adult hemoglobin by approximately 6 months postnatally. Except in a few thalassemia cases in which there may be a delay in cessation of Hb F production till 3 to 5 years of age.