Fetal Haemoglobin F

The blood is made up of different parts and each part has a different function. There are red blood cells – they carry oxygen around the body, white blood cells – help the body to fight infection, platelets – clots the blood and stops bleeding and plasma – which is mainly water.

The normal red blood cell is round, soft, spongy and pliable and resembles the shape of a doughnut that has been pressed in the middle slightly. Red blood cells are made in the bone marrow and live for 120 days approximately before they get destroyed and disposed of by the body but the body is constantly making new red blood cells.

The red blood cells contain a substance called haemoglobin, they are like tiny bubbles which combine with the air (oxygen) in the lungs, carry round the body to all the body parts to keep the body tissues and organs alive. Haemoglobin gives blood its red colour when it contains oxygen. These haemoglobin molecules (bubbles) stay freely flowing in the red blood cell, as illustrated in the picture below. There are different types of normal haemoglobin flowing in red blood cell, Adult haemoglobin, Fetal Haemoglobin and minor adult Haemoglobin A2 (A2).

The type of haemoglobin gene which a person has inherited from their parents determines the type of Adult haemoglobin they will have. Genes always come in pairs one from the mother and one from the father. Therefore, every individual inherits two Adult haemoglobin genes, see Haemoglobin.

The normal and most common haemoglobin gene combination that an individual can inherit from their parents is Haemoglobin AA (Hb AA).

During life in the womb and from very early in pregnancy every unborn baby produces Fetal Haemoglobin (Hb F), this is irrespective of the type of Adult Haemoglobin they have inherited from their parents.

At birth the level of Fetal Haemoglobin is high whilst the level of Adult Haemoglobin is low, but by one year of age there is a switch over between the level of Adult and Fetal haemoglobin.

For example, a person who has inherited normal adult Haemoglobin AA from their parents will have the following approximate Fetal, Adult and A2, see Haemoglobin A2, haemoglobin levels:

Haemoglobin Newborn > 1 Year Old to Adulthood
Haemoglobin F 90% <1%
Haemoglobin A 5 – 10% >95%
Haemoglobin A2 2.2 – 3.5% 2.2 – 3.5%

A person who has inherited Sickle Cell Trait (Hb AS) from their parents will have the following approximate Fetal, Adult and A2 haemoglobin levels:

Haemoglobin Newborn > 1 Year Old to Adulthood
Haemoglobin F 95% <1%
Haemoglobin A 5 – 10% >50%
Haemoglobin S 5 – 10% <50%
Haemoglobin A2 2.2 – 3.5% 2.2 – 3.5%

Fetal Haemoglobin is normal haemoglobin and is very efficient at carrying oxygen, even more efficient than adult haemoglobin, which is why it is very useful during fetal life when the unborn baby is totally dependent on its mother for oxygen.

Without Fetal Haemoglobin the unborn baby cannot develop or survive in the womb and such a fetus will die in the early stages of pregnancy.