Rhesus negative blood group in pregnancy

Rhesus factor is a substance (blood product) found in blood. 85% of the population has rhesus (Rh) positive (+) blood that is they have a presence of a substance in their blood. Approximately 15% of the population has rhesus (Rh) negative (-) blood that is they do not have a substance present in their blood. If you are pregnant and have rhesus negative blood (Rh-ve), there are some things you need to know if your husband/partner has rhesus positive (Rh+) blood.

How do I know my blood group?

When you first find out you are pregnant, your Lead Maternity Carer(LMC) will give you a form for some routine blood tests. One of the test is to find out your blood group and rhesus factor.

How can my negative blood be a problem?

If you are rhesus negative and your husband/partner is rhesus positive, then your unborn child may inherit negative or positive blood – you wont know until birth. There is usually a 60-70% chance that you baby will be rhesus positive. Therefore if you are negative and the baby happens to be positive it may cause problems for the baby as well as any future pregnancies you may have.

Problems occur if the baby's positive blood manages to find it’s way into your bloodstream, either during pregnancy or labour, mixing with your negative blood. If this happens and it is not treated, your blood can form antibodies to attack the positive blood being ‘foreign’, causing anaemia or in a worst case scenario, death for a baby. The formation of the antibodies is called Rhesus iso-immunization. This means that any future pregnancies you have where the baby is again rhesus positive, your antibodies may cross the placenta and attack the baby's red blood cells.

How can I prevent this?

Your LMC may organise for blood tests throughout your pregnancy to monitor antibodies in your blood. It is important that the LMC and the hospital where you birth your baby is aware of your rhesus negative blood, so they can test baby’s blood after the birth (from the placenta) to discover baby’s blood type. Should your baby have positive blood, you can choose to be given an “Anti-D” injection within 72 hours following the birth. This prevents antibodies from forming. If you experience any bleeding during pregnancy or if you have CVS, an amniocentesis, miscarriage, ectopic pregnancy or termination, you need to make your LMC aware of this as soon as possible, as all these may pose opportunities for the blood of the baby to mix with your blood and it is assumed that the baby is rhesus positive until blood test confirms otherwise following birth.

If none of the above things happen, in a normal pregnancy, you will not need ‘routine’ Anti-D injections until after the birth, should the baby’s blood taken from the placenta/cord come back as rhesus positive and if you choose to. Anti-D is a blood product and should be given following consent given by you. In New Zealand 'routine' or prophylaxis Anti D injections are not recommended in pregnancy.

Does having the Anti-D injection protect me for ever?

No – with each pregnancy the risk still exists of antibodies forming, should positive and negative blood become mixed. It is therefore important to make sure you continue to communicate your blood group with your future health practitioners if you decide to become pregnant again.

Can I have Anti-D if I have formed antibodies against positive blood?

If you have already formed antibodies against positive blood then the Anti-D injection may not be able to protect you or your baby. You will need to be closely monitored by the obstetrician and will possibly require specialist care. Discuss this with your LMC.