Rhesus negative and the anti-D injection

Pregnant woman having injection at doctors

You might not even know your blood type before you get pregnant – let alone your rhesus status – so why the sudden interest in the contents of your blood cells? Depending on your rhesus status and your baby's, your blood and your baby's blood may be incompatible.

When you have your booking appointment your blood will be tested for its rhesus status (among other things). If you are rhesus-D negative there could be health implications for any future pregnancies, but these can be avoided by an anti-D injection.

Why does it matter that my blood is rhesus negative?

Most people are rhesus-D positive, which means the surface of their red blood cells contains a protein called the D antigen. But around 15% of people in the UK are rhesus negative, known as RhD negative.

If you are RhD negative but your baby has inherited a rhesus positive status from its father, then your blood and your baby's blood may be incompatible. If your blood then comes into contact, an immune response is triggered and you will develop antibodies that fight against your baby's blood as though it were a foreign invader. This process is called sensitisation.

It's unusual for maternal and foetal blood to mix during pregnancy, and sensitisation does not normally occur until birth.
But once the antibodies are created, they do not leave your blood system and so, if you have a second pregnancy where your baby is rhesus positive, the antibodies can cross the placenta and attack and destroy the baby's blood cells causing rhesus disease.

Rhesus disease can be mild, causing anaemia, but it can also be very serious causing haemolytic disease of the foetus and newborn, which can be fatal.

How does sensitisation occur?

Sensitisation can occur spontaneously during pregnancy, but this only happens in about 1% of RhD negative women. It is more likely to happen during a 'sensitising event', such as the following:

  • Bleeding during pregnancy
  • Invasive procedures such as an amniocentesis or CVS
  • Abdominal injury during pregnancy
  • External cephalic version (where the baby is turned in the later stages of pregnancy)

It can also occur during a termination, miscarriage, ectopic pregnancy or blood transfusion. It is important that you let your midwife know if you are RhD negative and have experienced any of these events.

If you are RhD negative, you will be tested for the production of antibodies at 28 weeks and if you have them your pregnancy will be monitored carefully.

If sensitisation occurs during birth, the initial immune response takes a while to kick in and it doesn't usually have time to harm the baby. However, the antibodies are much quicker to activate when they next come into contact with rhesus positive blood, so there are greater risks for subsequent pregnancies.

Anti-D injection

Everyone told me it was an awful injection to have, but I had one of them today and it wasn't that bad.

The good news is that there is a simple way to prevent sensitisation during pregnancy. Anti-D immunoglobulin can be given whenever there is a risk of sensitisation (such as after a fall, bump or invasive medical procedures). It can also be given routinely in the third trimester for all women who are RhD negative. This is not policy throughout the NHS, but if your area does offer this, it will probably be given at week 28 and week 34.

Anti-D neutralises the D antigen in foetal blood so maternal blood (ie yours) does not detect it and produce antibodies to fight it. The injections can be given by your midwife.

After your baby is born, a blood sample from the umbilical cord can be taken and tested for the D antigen. If the baby is RhD positive, another anti-D injection should be given within 72 hours to minimise the risk of sensitisation.

Is Anti-D safe?

Anti-D is derived from donor blood, so it carries a very small risk of blood-borne infection. The risk is thought to be one in 10,000 billion doses – ie pretty low odds. If you have objections to receiving blood projects, you can refuse the anti-D injection.

It may cause a localised allergic reaction, but again this is rare. Women are asked to stick around for 20 minutes after the injection in case this happens.

What Mumsnetters say about being rhesus negative and the anti-D injection

  • “It's about protecting the next baby, not this one.”
  • “It's very important to protect you and baby – it can cause problems if you produce antibodies against the baby – plus it will matter if you want more children as well.”
  • “The rhesus thing isn't a big deal but your midwife should be aware of it, and you'll need to let them know if you sustain an impact (but you would anyway).”
  • “I'm rhesus negative. It's a minority blood group, so unless your partner is also RhNeg, there is a high chance your baby is a different blood group. The good news is that these days it is no big deal. They will offer you prophylactic (just in case) anti-immunoglobulin to prevent any problems. You should also tell your midwife if you have any bleeding or any kind of accident that could have caused a small internal bleed without you realising it.”