Your baby’s blood group may be different from yours and occasionally the two aren’t compatible. Dr Howard Lee explains the Rhesus Negative Factor in pregnancy

We are what we are because of our genes. We inherit pairs of them from both our parents at the moment of conception. Some genes are more ‘dominant’ than others, and our rhesus state is one of them.

At your first antenatal screening, blood tests are taken in order to determine your blood type (A, B, AB or O) and your rhesus status (Rh-positive or Rh-negative).

• If you have the rhesus factor (which is a protein on the surface of your red blood cells) you are Rh-positive, if you don’t you are Rh-negative. Most people (about 85%) are Rh-positive.
• The rhesus state only matters in pregnancy if the mother is Rh-negative, the father is Rh-positive and the baby is also Rh-positive.

There are, in fact, various rhesus genes. Among them are c, d and e, which can be either positive (C, D, E) or negative (c, d, e). It is the ‘d’ genes that particularly concern us.

Rh(D) positive cells contain a substance (D antigen) which can stimulate Rh(d) negative blood to produce harmful antibodies that destroy red cells. The harmful antibody is called ‘anti-D’ and can be produced if a mother is Rh-negative but her baby is Rh-positive.

Rhesus incompatibility doesn’t occur with first pregnancies because the antibodies aren’t present in the mother’s blood. However, in subsequent pregnancies, if the babies are rhesus positive, there may be a problem. The mother’s antibodies will cross over the placenta into the baby’s blood and, regarding it as ‘foreign’, will try to break it down.

This causes problems with the baby’s haemoglobin level (the iron-carrying element in the red blood cells) which then falls, causing anaemia. Blood transfusion are then necessary at birth and babies could also be severely jaundiced.

Rhesus disease is now rare and poses little threat because any complications can be anticipated and treated.

Treatment is given if:

• An exchange of blood between mother and baby takes place during delivery
• A rhesus-negative mother has had bleeding during pregnancy
• The mother has had a miscarriage, which can cause her blood to produce antibodies attacking the Rhesus positive blood of her lost baby
• Other situations – chorionic villus sampling, amniocentesis or ectopic pregnancy – have lead to contact with foetal blood

These injections will prevent your blood from producing antibodies against Rh(D) blood cells, thus protecting you during any future pregnancies.

Screening during pregnancy

If your screening blood test shows that you are rhesus negative, you will have extra blood tests during pregnancy to see if you have developed any rhesus antibodies. This rarely happens, but if it does, you will be carefully monitored.

• You will also be given an injection of Anti-D immunoglobulin between your 28th and 29th weeks of pregnancy.
• After delivery, if your newborn is Rh(D) positive (this will be determined from an umbilical cord blood sample), you will be given another injection within 72 hours.

Anti-D injections are prepared using donor blood possessing high amounts of antibodies and there are rarely any side effects. Any risks are far outweighed by the enormous benefits of Anti-D injections.