What's the big deal if I'm Rhesus negative?

At your first antenatal clinic appointment, blood samples are taken in order to determine your blood group (A, B, AB or O) and to see if you are Rhesus positive or Rhesus negative. You are Rhesus positive if you have the Rhesus factor (which is a protein on the surface of your red blood cells). If the protein is not present you are Rhesus negative. Most people (about 85% of Caucasians and up to 94% of the Asian or African population) are Rhesus positive.

The Rhesus state only matters in pregnancy if the mother is Rhesus negative, the father is Rhesus positive and the baby is also Rhesus positive. However, we will not know if the baby is Rhesus positive or negative until it is born, and, in the UK we often don’t know the blood group and Rhesus status of the father, so all Rhesus negative mothers are treated routinely.

Rhesus positive cells contain a substance which can stimulate Rhesus negative blood to produce harmful antibodies that destroy red cells. This can be produced if a mother is Rhesus negative but her baby is Rhesus positive. This is called Rhesus incompatibility.

Rhesus incompatibility doesn’t happen with first pregnancies because the antibodies aren’t present in the mother’s blood. However, in later pregnancies, if the babies are Rhesus positive, there may be a problem. This is because the mother may have started producing the antibodies, which will cross over the placenta into the baby’s blood and, regarding it as ‘foreign’, will try to break it down.

In the UK an injection of anti-D is given routinely to Rhesus negative pregnant mothers at 28 and 34 weeks, to stop possible production of the harmful antibody, just in case.

Treatment by injection of anti-D is also given to a Rhesus negative mother if:

• an exchange of blood between mother and baby takes place during delivery
• she 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 have led to contact with her baby’s blood during pregnancy – such as chorionic villus sampling, amniocentesis or ectopic pregnancy

After delivery, your newborn’s Rhesus status will be checked from an umbilical cord blood sample at birth. If your baby is Rhesus positive, you will be given another injection within 72 hours. This will then stop you from building up the antibodies in case any future pregnancies are with Rhesus positive babies.

Although this all seems very confusing remember that it is commonly checked and well controlled, so Rhesus incompatibility is now very rare.