Hemoglobin is the oxygen-carrying substance in red blood cells. Fetal hemoglobin (HbF) is the type of hemoglobin produced by a fetus's body.

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

With certain diseases, such as sickle cell disease and thalassemia, some people continue to produce HbF.