Hemoglobin is the oxygen-carrying substancein red blood cells. Fetal hemoglobin (HbF) is the type of hemoglobin producedby a fetus's body.

At birth, babies typically have half fetal hemoglobin andhalf adult hemoglobin (HbA). After birth, HbF is gradually replaced by HbA.

Some people with sickle cell disease continue to produce someHbF, which helps protect them from more severe complications ofthe disease.