What determines our blood group?
Our blood group is determined by the combination of certain proteins (antigens) on the surface of our red blood cells (there are about 300 different types of these proteins!). The most common ones you have probably heard of are the ones are those that give us our A-B-O group and our Rhesus status (positive or negative).
Why is a mother’s blood group and rhesus status significant in pregnancy?
It is a similar situation to when someone receives a blood transfusion. If the blood types are not compatible, then the person receiving blood will develop an immune response to the foreign red blood cells, essentially treating them the same as if they were a viral or bacterial infection.
From about 6 weeks of pregnancy, the developing baby’s blood cells will have their surface proteins present. Let’s consider the Rhesus-D antigen. If a mother is Rhesus negative, it means that her red blood cells don’t have this protein. If her baby is Rhesus positive (because the father is and has passed on this gene) then, as baby’s blood cells find their way into the mother’s circulation, they will be recognised as foreign. The mother’s immune system will make special proteins called immunoglobulins (Ig for short) which cover the foreign cells and help the body get rid of them.
In the first pregnancy, this doesn’t matter so much as the initial Ig that are produced are large and don’t readily cross the placenta. In future pregnancies, however, if a mother has been sensitised and is carrying another Rh positive baby, then the immune response will be much greater and the new Ig (which are much smaller) will cross the placenta. They will then attack baby’s own red blood cells and can cause severe anaemia and even loss of the pregnancy.
Can anything be done to prevent this?
Yes. In any woman who is Rh negative, we can use an immunoglobulin (Anti-D) given by an injection to prevent her becoming sensitised. This works by the injected Ig coating any of baby’s red cells floating around mum’s blood stream and effectively hiding them from her immune system. Anti-D is typically given at 28 and 34-36 weeks of pregnancy, or at any other time there is a chance of exposure (e.g. vaginal bleeding, amniocentesis).
Do all Rh negative women need Anti-D?
If the father of the baby is also Rh negative, then there is no possibility of the baby being Rh positive. In this case, no Anti-D is required. In cases of uncertainty, then testing of baby’s blood cells floating in the mother’s blood stream can be performed to determine baby’s blood type and antigen status.
What happens after birth?
After the baby is born, its blood type is confirmed. If the baby is Rh negative, no further Anti-D is required. If the baby is Rh positive, then a further dose is given to the mother in the day or two after delivery.
Is the D antigen the only one that can cause problems in pregnancy?
No. There are many other red cell antigens, some of which can cause similar problems to the D antigen, but the D antigen is the only one we can prevent causing problems.
What happens if a pregnancy is affected by antigen incompatibility?
If this occurs, we can monitor the level of Anti-D (or any other antigen) in mum’s blood stream to see if it is rising. The levels are used in combination with ultrasound of the arteries in baby’s head to determine if baby is being affected. Sometimes babies may need therapy while still in the womb, or may require early delivery.
The above information doesn’t take the place of a medical consultation so please seek further advice if you have specific questions or concerns.