Pregnancy blood tests - the Rhesus factor and disease prevention

Pregnancy blood tests: As part of your antenatal care, you will be offered a number of blood tests. Some are offered to all women and some are only offered if it is thought that you are at risk of a particular infection or inherited condition. All of the tests are done to help make your pregnancy safer or to check that your baby is healthy. Talk to your midwife or doctor so that you understand why the blood tests are being offered and so that you can make an informed choice about whether or not you want them. Your midwife or doctor should also inform you about the tests. Below is an outline of all the tests that can be offered.

Blood group and Rhesus factor test    

Your blood will be tested to check your blood group and to see whether you are rhesus negative or positive. Some women are rhesus negative. This is usually not a worry for a first pregnancy but it may affect the next child.

People who are rhesus positive have a substance known as D antigen on the surface of their red blood cells. Rhesus negative people do not.

A woman who is rhesus negative can carry a baby who is rhesus positive if the baby’s father is rhesus positive. During pregnancy or birth, small amounts of the baby’s blood can enter the mother’s bloodstream. This can cause the mother to produce antibodies. This usually doesn’t affect the existing pregnancy, but the woman becomes ‘sensitized’. This means that if she gets pregnant with another rhesus positive baby, the immune response will be quicker and much greater. The antibodies produced by the mother can cross the placenta and attach to the D antigen on her baby’s red blood cells. This can be harmful to the baby as it may result in a condition called haemolytic disease of the newborn, which can lead to anemia and jaundice.

Prevention of rhesus disease

Anti-D injections prevent rhesus negative women producing antibodies against the baby and reduce the risk of a rhesus negative woman becoming sensitized.

Rhesus negative mothers who are not sensitized are offered anti-D injections at 28 and 34 weeks as well as after the birth of their baby.

This is quite safe for both the mother and her baby.