Rhesus negative: what does it mean in pregnancy?
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During your first antenatal appointment you'll be offered several blood tests. One of these is to check your blood group and the other determines your rhesus status (positive or negative). So what does it mean for you and your baby if you are rhesus negative?
Why is your rhesus status important during pregnancy?
Your rhesus status is set in your genes - around 15% of people in the UK are rhesus negative.
So what does this mean? People who are rhesus positive have a substance known as D antigen on the surface of their red blood cells. If you're rhesus negative (RhD negative), you don't have this.
This rhesus status isn’t usually a concern for a first pregnancy. However there could be health implications for any future pregnancies if an immune response called sensitisation occurs.
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What is sensitisation?
Sensitisation occurs when a rhesus negative mother is exposed to rhesus positive blood and her body creates antibodies to destroy what it views as foreign blood cells.
This can happen if you are rhesus negative and your baby is rhesus positive. Your immune system could react to the D antigen in your baby's blood if it enters your bloodstream. Your body will then have an immune response and produce antibodies against it, a process called sensitisation.
There are various ways your baby's blood can get into your bloodstream:
- Vaginal bleeding
- Miscarriage
- Termination
- Ectopic pregnancy
- Blood transfusion
- Injuries to the abdomen - such as a blow to the stomach or an accident
- Invasive tests in pregnancy, for example an amniocentesis or chorionic villus sampling
It's worth nothing that sensitisation is not usually harmful if it is your first pregnancy.
However it can be an issue in any future pregnancies if your baby is rhesus positive. This means the antibodies your body made during the sensitisation in your first pregnancy could multiply much quicker. These antibodies could then attack your baby's red blood cells.
But don’t worry, sensitisation is easily avoided with an anti-D injection which stops your blood producing these antibodies.
What is the anti-D injection?
All rhesus negative women will be offered an anti-D injection, which helps to minimise the risks of sensitisation by neutralising any RhD positive antigens that may have entered the mother’s blood.
It is given as either a one-dose treatment - between weeks 28 to 30 - or a two-dose treatment - one on week 28 and another on week 34.
There has been no proven difference in effectiveness between the one-dose or two-dose treatments.
If you have already been sensitised the anti-D injection won’t work. Instead you will be closely monitored so treatment can begin if problems develop.
Is the anti-D injection safe?
The anti-D injection is made from the blood plasma of selected donors, so it carries a very small risk of blood-borne infection. However this risk is tiny.
Some women may develop a slight allergic reaction, which can include a rash or flu-like symptoms, but again, the risk of this happening is very low.
You can refuse the anti-D injection if you wish, but the evidence shows that the benefits of preventing sensitisation far outweigh the small risks.
You may not want to have the injection if:
- You don’t plan to have any other children
- You plan to be sterilised after giving birth
- You are certain your baby’s father is also rhesus negative. This means your baby will then be rhesus negative too.
