BLOOD GROUP TESTS AND PREGNANCY

Date: Monday, March 23rd, 2009 at 3:45 am

One of the few times it is important to know your blood group is in pregnancy. If you are giving blood, it is important the Blood Bank knows, too, but most of us can get by without knowing. If ever a person is in an accident, and requires an emergency blood transfusion, blood will be taken for testing at the time. In pregnancy, however, it is tested because of a thing called the Rhesus or Rh factor.

All our blood cells have characteristics which make our blood either type A, B, O, or AB. These characteristics are inherited, like eye colour, through our genes. No one type works better than any other.

The Rh factor is another inherited marker on our blood cells, and there are two possibilities, Rh positive and Rh negative. Again, it makes no difference to the functioning of the cells.

So a person could have a blood group A, Rh positive, or AB, Rh negative, or any combination, depending on what they inherited from their parents. About 15 per cent of our population are Rh negative.

A pregnant woman should have her blood tested at the start of the pregnancy. She will also have a test to see if there is any evidence of blood group antibodies. Antibodies are the things made by the body’s immune system to fight bugs. Sometimes the immune system will recognise blood cells with different characteristics (like a different blood group) as foreign, in the same way as they recognise bugs as foreign. The immune system makes antibodies to try to destroy these ‘foreign’ cells, and the antibodies stay in the blood, and can fight any more of the same trespassing cells if need be.

If a pregnant woman’s blood group is Rh negative, it means her body will recognise Rh positive blood as foreign, and make antibodies against it. An Rh negative woman can have an Rh positive foetus in her uterus. Even though the woman’s blood does not usually mix with that of her foetus, occasionally there may be some spillage from the foetal circulation into the woman’s. If this happens, antibodies may be made against the foetal blood.

The times that mixing of the blood is more likely to occur are if there is any bleeding during pregnancy, or at delivery. At these times, an Rh negative woman can be given a special injection of a substance called anti-D. If this is given within seventy-two hours of the blood mixing, it can prevent the woman making Rh antibodies.

Rh antibodies may not affect the current pregnancy, but if a woman develops antibodies in one pregnancy, it is likely that they will have more of an effect in future pregnancies. The effect they can have is to act against the foetal red blood cells, and destroy them. This may have only a minor effect on the foetus or, if there are high levels of antibody, it may make the foetus very sick and anaemic. If it is recognised during pregnancy, Rh immunisation, as it is known, can be monitored, and even treated. Fortunately, because of the use of anti-D, this is rarely needed these days.

Fresh blood group antibody tests are needed each pregnancy, even if you know your blood group, to check if antibodies have been made against any of the other subgroups, apart from the Rhesus factor. This is uncommon, but worth checking as it too can be monitored and treated.