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Red blood cells sometimes contain a protein known as the RhD antigen. If you have this protein, your blood group is RhD-positive. If not, your blood group is RhD-negative.
Pregnant women are given a blood group test. This is because if the mother is RhD-negative but the child has inherited RhD-positive blood from the father, it could cause complications if left untreated. RhD-negative women of child-bearing age should only receive RhD-negative blood.
Which blood group causes problems in pregnancy?
The "positive" or "negative" part of your blood type, such as O positive or A negative, refers to your RhD status. During pregnancy, problems can occur if you are RhD-negative and your foetus is RhD-positive. We can give you treatment to prevent these problems.
Rhesus disease is a condition where antibodies in a pregnant woman's blood destroy her baby's blood cells. It's also known as haemolytic disease of the foetus and newborn (HDFN).
Rhesus disease doesn't harm the mother, but it can cause the baby to become anaemic and develop newborn jaundice.
What causes rhesus disease?
Rhesus disease happens when the mother has rhesus-negative (RhD-negative) blood and the baby in her womb has rhesus-positive (RhD-positive) blood. The mother must have also been previously sensitised to RhD-positive blood.
Sensitisation happens when a woman with RhD-negative blood is exposed to RhD-positive blood, usually during a previous pregnancy with an RhD-positive baby. The woman’s body responds to the RhD-positive blood by producing antibodies (molecules which fight infection) that recognise the foreign blood cells and destroy them.
If sensitisation occurs, the next time the woman is exposed to RhD-positive blood, her body produces antibodies immediately. If she's pregnant with an RhD-positive baby, the antibodies can cross the placenta, causing rhesus disease in the unborn baby. The antibodies can continue attacking the baby's red blood cells for a few months after birth.
Preventing rhesus disease
Rhesus disease is uncommon these days because it can usually be prevented using injections of a medication called anti-D immunoglobulin (anti-D).
All women are offered blood tests as part of their antenatal checks, including tests to determine whether their blood is RhD-negative or positive. If the mother is RhD-negative, she'll be offered injections of anti-D at certain points in her pregnancy when she may be exposed to the baby's red blood cells. This helps to remove the RhD foetal blood cells before they can cause sensitisation.
If a woman has developed anti-D antibodies in a previous pregnancy (she's already sensitised) then these immunoglobulin injections don't help. The pregnancy will be monitored more closely than usual, as will the baby following birth.
Treating rhesus disease
If an unborn baby does develop rhesus disease, treatment depends on how severe it is. In more severe cases, the unborn baby may need a blood transfusion. After delivery, the child is likely to be admitted to a neonatal unit (a hospital unit that specialises in caring for newborn babies).
Treatment for rhesus disease after delivery can include a light treatment called phototherapy, blood transfusions, and an injection of a solution of antibodies (intravenous immunoglobulin) to prevent red blood cells being destroyed.
If rhesus disease is left untreated, severe cases can lead to stillbirth. In other cases, it could lead to brain damage, learning difficulties, hearing loss and blindness and vision loss. However, treatment is usually effective and these problems are uncommon.
Blood tests
A blood test should be carried out early on in your pregnancy to test for conditions such as iron deficiency anaemia, rubella (german measles), HIV and AIDS, and hepatitis B.
Your blood will also be tested to determine which blood group you are, and whether your blood is RhD-positive or RhD-negative.
If you're 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 and you'll be offered an injection of a medication called anti-D immunoglobulin (anti-D) to reduce the risk of your baby developing rhesus disease.
If anti-D antibodies are detected in your blood during pregnancy, there's a risk that 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. This is because your baby won't be at risk of rhesus disease if both the mother and father have RhD-negative blood.
Checking your baby's blood type
It's 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's usually possible to get a reliable result from this test after 11 to 12 weeks of pregnancy, which is long before the baby is at risk from the antibodies.
If your baby is RhD-negative, they're not at risk of rhesus disease and no extra monitoring or treatment will be necessary. If they're found to be RhD-positive, the pregnancy will be monitored more closely so that 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 routinely, to see if they're carrying an RhD-positive or RhD-negative baby, to avoid unnecessary treatment.
Last reviewed: 01 March 2023