Why does symptoms of beta thalassemia appear at birth?

Symptoms usually don't appear at birth because infants have high levels of fetal hemoglobin (HbF) which can compensate for the absence or reduction of adult hemoglobin (HbA) in beta-thalassemia. Fetal hemoglobin is able to carry oxygen effectively during the fetal stage and the first few months after birth. As the baby grows older and the production of fetal hemoglobin decreases, the deficiency of adult hemoglobin becomes more apparent and the symptoms of beta-thalassemia start to manifest, typically between the ages of 6 months to 2 years.