​Fetal hemoglobin (hemoglobin F) is the main hemoglobin that carries oxygen around the body of the developing baby during the last 7 months of pregnancy.

• Fetal hemoglobin is best suited to the conditions in the womb and the oxygen transport needs of babies before they are born.
• A few weeks before birth the baby starts to make increasing amounts of adult hemoglobin (hemoglobin A). Hemoglobin A is better suited to the oxygen transport needs after birth and throughout adult life. After birth the baby makes less and less hemoglobin F and more and more hemoglobin A.
• Hemoglobin F does not turn into hemoglobin A. Hemoglobin F and hemoglobin A are completely different hemoglobins.
• Hemoglobin is made up of 4 proteins chains and a "heme" group that contains iron. Hemoglobin F is made up of 2 alpha chains and 2 gamma (fetal) chains. Hemoglobin A is made up of 2 alpha chains and 2 beta (adult) chains.
• As they grow babies automatically turn off the production of hemoglobin F and turn on the production of hemoglobin A- It takes about 2 years for a baby to completely switch over to adult hemoglobin.
• A baby who makes normal fetal hemoglobin will not necessarily be able to make normal adult hemoglobin.
• Sickle cell disease is caused by an abnormal adult hemoglobin, called hemoglobin S. People with sickle cell disease make hemoglobin S instead of hemoglobin A.
• Newborn babies with sickle cell disease make hemoglobin F and hemoglobin S. Most babies (who don't have sickle cell disease) make hemoglobin F and hemoglobin A.
• Babies with sickle cell disease experience more problems as hemoglobin F is turned off. Most people with sickle cell disease made normal hemoglobin F as babies.
• Hydroxyurea, one of the new drugs used to treat sickle cell disease in adults works by turning hemoglobin F back on.