Haemolytic disease of the newborn

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Haemolytic disease of the newborn is caused when the blood group of the mother and baby are not compatible.

People can belong to one of four blood groups, A, B, AB and O, which are carried on the red blood cells. The other important difference in people's blood is called rhesus factor, which is also found in the red blood cells. People who are rhesus positive have a substance known as D antigen on the surface of their red blood cells. People who are rhesus negative do not have the D antigen on their blood cells.

During pregnancy and childbirth the rhesus factor D can have an important role to play when a RhD-negative mother is expecting a RhD-positive baby. The placenta acts as a barrier between the red blood cells of the mother and baby. However, sometimes small amounts of the baby's blood can cross over into the mother's blood stream.

If any of the blood cells from a RhD-positive baby get into the blood of a RhD-negative woman, she will react to the D antigen in the baby's blood as though it is a foreign substance and will produce antibodies. This is called 'rhesus sensitisation'.

As a general rule, the first child that triggers this sensitisation does not suffer any adverse consequences, as it will already have been born by the time antibodies have developed. However, if the woman goes on to become pregnant with a second RhD-positive child, antibodies may cross into the baby's blood and attack the red blood cells. This is called 'haemolytic disease of the newborn'.

Haemolytic Disease of the Newborn can be mild but can also lead to jaundice, brain damage or even the death of the child. With further RhD-positive children, the antibody reaction becomes faster and more serious.