Updated 19 February 2016

Rhesus incompatibility

Rhesus is an inherited blood group protein, or antigen.


The Rhesus, or Rh factor is an inherited blood group protein, or antigen found on the surface of red blood cells. If your red blood cells express Rh (D), you are considered to be Rh-positive, otherwise as Rh-negative if it is absent.

If an Rh-negative woman expects the baby of an Rh-negative father, the baby has a 100 percent chance of being Rh-negative, and all will be well. However, if the baby's mother is Rh-negative and the father Rh-positive, chances are either 100 percent or 50 percent (depending on the father's genetic constitution) for the baby to be Rh-positive, which could lead to Rh incompatibility.

What is Rh incompatibility?

The Rh-positive blood cells from the foetus (unborn baby) could pass into the mother's bloodstream during pregnancy or at birth, where they would be recognised as a foreign substance, in which case, the mother will produce antibodies against the foetus' red blood cells. If this is the mother's first pregnancy, she will have little or few of these antibodies and they will be unlikely to affect the foetus, however, there will be a strong chance that future Rh-positive foetuses will be harmed if the mother is not treated accordingly. The result for the foetus may be haemolysis (breakage of red blood cells), jaundice, anaemia, heart failure, even intra-uterine death, or brain damage after birth resulting from severe jaundice.

How is Rh incompatibility treated?

The Rh-negative mother's blood should be checked every two months during successive pregnancies for these antibodies, especially after any activity which may cause the mother's and foetus' blood to mix, such as after amniocentesis or chorionic villus sampling (CVS), and within the first 72 hours after delivery. At these times, the mother must be given a Rh(D) immune-globulin injection (Rhogam) to destroy these antibodies should they exist. This injection is also necessary, should the pregnancy be terminated through an abortion or miscarriage.


It is particularly important for all expectant mothers to know their blood's Rh factor, especially if it is Rh negative. The baby's life could be in danger if the mother's antibodies, formed against the foetus' Rh-positive blood cells, attack these cells. If this happens, an exchange transfusion, in which the foetus' blood is exchanged for new blood that matches the mother's, may save the baby's life.

The mother will produce antibodies if she is exposed to the blood from an Rh-positive baby during childbirth or when the placenta tears and some of the baby's blood enters her bloodstream. Therefore, the formation of antibodies can often be prevented if the mother is given anti-Rh antibodies to suppress the formation of antibodies in her blood after the birth of her first Rh-positive child.

Donor compatibility

When a blood transfusion is necessary, donor and recipient blood must be compatible with regard to major ABO group and Rh type. If not, in the case of ABO bloodgroup, the antibodies in the recipient's blood will react immediately against the incompatible donor blood cells, causing the red cells of the donor blood to clump together (agglutinate), then burst in the blood stream (haemolysis). This can be fatal. In the case of Rh bloodgroup, if Rh positive red cells are transfused into a Rh negative recipient, the recipient will be sensitised and form Rh antibodies (similar to the Rh-negative mother exposed to the baby's blood in the first pregnancy). When the Rh-negative recipient is exposed a second time to Rh-positive blood, a haemolytic transfusion reaction will occur.

Normally patients will receive blood with the same Rh and ABO group. In an emergency, a patient with a different blood group may be given O negative blood, because it most likely to be accepted by all blood types. However, there is still a risk involved. In general it is best to only mix blood of matching blood groups for ABO and Rh types.


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