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Rhesus disease is a cause of haemolysis manifest in the first 24 hours of life due to rhesus incompatibility between mother and baby. It is the result of a mother being rhesus negative and having antibodies produced towards a rhesus positive baby. It occurs after the mother has been sensitised by either a mismatched blood transfusion, or from foetal blood entering her circulation during miscarriage, abortion, placental bleeding, amniocentesis or external cephalic version. Most commonly it occurs at the end of a previous pregnancy during labour and delivery.
In all these instances the sensitising blood has been rhesus positive. The mother reacts to fetal blood by producing antibodies of anti-Rh D type, which cross the placenta during pregnancy and cause haemolysis of the fetal red cells. Sensitisation is more likely if the mother and foetus are ABO compatible, as this ensures that fetal cells persist in the maternal circulation for a more potent immune reaction to be stimulated.
The severity of the condition caused by Rhesus incompatibility varies from the baby born with mild jaundice and anaemia to the development of hydrops fetalis in utero. The latter is usually fatal.
Note that Rhesus D antigen is the most important and its absence is used to categorise Rhesus negative. However antibodies to other Rhesus antigens may develop (Rh C, Rh E), and can cause Rhesus immunisation.