Why are there no symptoms of beta thalassemia at birth?

Beta gene not needed: Newborns have fetal hemoglobin, not adult hemoglobin, in their red blood cells. Beta thalassemia major babies look normal at birth because fetal hemoglobin doesn't need beta globulin from the beta genes. As fetal hemoglobin goes away after birth and adult hemoglobin starts being made, the baby will feel the effects of his lack of beta globulin. Symptoms start after the first 6-8 months of life.

Fetal hemoglobin: Beta thalassemia is expressed differently based on how many abnormal gene copies are present at birth. Many infants with abnormal Hemoglobin chains at birth will have an asymptomatic period of 3-6 months due the predominance of fetal Hemoglobin at birth. Fetal hemoglobin over time is replaced by the defective beta chains over the first few months of life, causing classic symptoms