Why maternal blood group matters in pregnancy

Blood group typing in pregnancy is important for various reasons. Standard blood groups are A, B, AB or O. Each group is additionally labelled either positive (+) or negative (-), which denotes what is called Rhesus factor. The Rhesus factor is a marker on the red blood cells, and majority of people have this factor and are labelled Rhesus positive. However, about one in every ten people are Rhesus negative.

Rhesus negativity in the mother, with a positive father can cause potential problems in pregnancy. The unborn baby inherits blood group and Rhesus factor from either parent, or in combination. If the mother is Rhesus negative and the father positive, the possibility of the baby being Rhesus positive arises, and precautions must be taken to negate potential complications. For practical purposes, the father is usually deemed Rhesus positive once the mother is proven to be Rhesus negative.

If the unborn baby inherits the father’s Rhesus factor and becomes positive, the potential for problems arises. Any cross-over of the unborn baby’s blood into the mother provokes formation of antibodies against the baby’s red blood cells. Such antibodies then cross back through the placenta into the baby, and destroy the unborn baby’s red blood cells. The baby develops anemia whilst still inside the uterus, and in severe cases this leads to serious problems including heart failure. This is called Rhesus disease, and was often fatal in the older days.

So what happens if an expectant mother is Rhesus negative? A blood test is usually done in the second half of the pregnancy to confirm that the mother does not already have antibodies to the Rhesus factor. Then an injection called Anti-D is given to reduce the risks of the mother forming antibodies. Anti-D is 99% effective, and over the years has considerably reduced the number of babies affected by Rhesus disease. Alternatively, novel techniques of confirming the unborn baby’s blood group can be instituted thus avoiding Anti-D if the baby is confirmed to be Rhesus negative. And if both partners are Rhesus negative, nothing further needs to be done.

Rhesus disease is uncommon in first pregnancies as it takes time for the mother to form antibodies. However the risk increases in subsequent pregnancies, and if there is bleeding in pregnancy or the mother undergoes some invasive procedures. Abortions and ectopic pregnancies increase the chance of antibodies being formed, and Anti-D is advised in such situations. Following delivery, the mother usually needs an extra dose of Anti-D if the baby’s blood group turns to be Rhesus positive.

If the expectant mother has already formed antibodies, Anti-D is not useful. The pregnancy must be closely monitored usually by blood tests and ultrasound scans. Severely affected babies may need blood transfusions whilst still in the uterus, and are usually delivered early.