Rhesus factor disease - Diagnosing rhesus disease

The routine screening tests offered while you're pregnant can usually determine if your baby is at risk of rhesus disease.

Blood tests

A blood test should be carried out early on in your pregnancy to test for conditions such as anaemia, rubella, HIV and hepatitis B.

Your blood will also be tested to determine which blood group you are, and whether your blood is rhesus (RhD) positive or negative (see causes of rhesus disease for more information).

If you are RhD negative, your blood will be checked for the antibodies (known as anti-D antibodies) that destroy RhD positive red blood cells. You may have become exposed to them during pregnancy if your baby has RhD positive blood.

If no antibodies are found, your blood will be checked again at 28 weeks of pregnancy. If there are still no antibodies present at this point, you will offered an injection of a medication called anti-D immunoglobulin to reduce the risk of your baby developing rhesus disease (see preventing rhesus disease for more information).

If anti-D antibodies are detected in your blood during pregnancy, there is a risk your unborn baby will be affected by rhesus disease. For this reason, you and your baby will be monitored more frequently than usual during your pregnancy.

In some cases, a blood test to check the father's blood type may be offered if you have RhD negative blood because your baby will not be at risk of rhesus disease if both the mother and father have RhD negative blood.

Checking your baby's blood type

It is now possible to determine if an unborn baby is RhD positive or RhD negative by taking a simple blood test during pregnancy. 

Genetic information (DNA) from the unborn baby can be found in the mother's blood which allows the blood group of the unborn baby to be checked without any risk. It is usually possible to get a reliable result from this test after 11-12 weeks of pregnancy, which is long before the baby is at risk from the antibodies.

If your baby is RhD negative, they are not at risk of rhesus disease and no extra monitoring or treatment will be necessary. If they are found to be RhD positive, the pregnancy will be monitored more closely so any problems that may occur can be treated quickly.

In the future, RhD negative women who haven't developed anti-D antibodies may be offered this test to see if they are carrying an RhD positive or RhD negative baby, to avoid unnecessary treatment.

Monitoring during pregnancy

If your baby is at risk of developing rhesus disease, they will be monitored by measuring the blood flow in their brain. If your baby is affected, their blood may be thinner and flow more quickly. This can be measured using a type of ultrasound scan called a Doppler ultrasound.

If a Doppler ultrasound shows your baby’s blood is flowing faster than normal, a procedure called fetal blood sampling (FBS) can be used to check whether your baby is anaemic. This procedure involves inserting a needle through your abdomen (tummy) to remove a small sample of blood from your baby. The procedure is performed under local anaesthetic, usually on an outpatient basis so you can go home on the same day.

There is a small (usually 1-3%) chance that this procedure could cause you to lose your pregnancy, so it should only be carried out if necessary.

If your baby is found to be anaemic, they can be given a transfusion of blood through the same needle. This is known as an intrauterine transfusion (IUT) and it may require an overnight stay in hospital.

FBS and intrauterine transfusion are not available in all hospitals, so you may need to be referred to a different hospital to the one where you were going to have your baby. 

Read more about treating rhesus disease.

Diagnosis in a newborn baby

If you are RhD negative, blood will be taken from your baby’s umbilical cord when they are born to check their blood group and to see if the anti-D antibodies have been passed into their blood. This is called a Coombs test.

If you are known to have anti-D antibodies, your baby’s blood will also be tested for anaemia and jaundice.