As part of your pregnancy, you should be offered several routine screening tests. Before having a screening test your GP or midwife will discuss it with you. You don't have to have a test if you don't want it.
Blood testing
A blood test will be used early on in your pregnancy so that the sample can be tested for conditions such as:
Your blood will also be tested to determine which ABO blood group you are, and whether your blood is rhesus (RhD) positive or negative.
If you are RhD negative, a test will be carried out to make sure your blood is not producing antibodies (known as anti-D antibodies). This rules out the possibility that you have already been sensitised. Your blood will be checked again at 28 weeks of pregnancy. You will be offered routine administration of anti-D prophylaxis to help prevent sensitisation occurring.
Diagnosis in an unborn baby
If you develop anti-D antibodies during your pregnancy, or if you have antibodies present from a previous pregnancy, there is a risk your baby will be affected and become anaemic before it is born. For this reason, you and your baby will be monitored more frequently than usual during pregnancy.
Your baby will be monitored by measuring the blood flow in their brain. If your baby is anaemic, their blood will be thinner and flow more quickly. Your baby’s blood flow can be measured using a type of ultrasound scan called a Doppler ultrasound.
If a Doppler ultrasound shows that your baby’s blood is flowing at a higher speed than normal, a procedure called foetal blood sampling (FBS) can be used to check whether your baby is anaemic. In FBS, a needle is inserted through your abdomen and is used to take a small sample of blood for testing, either from your baby’s umbilical cord, or from the umbilical vein as it passes through their liver. The procedure is performed under local anaesthetic, usually on an outpatient basis (where you do not have to stay in hospital overnight).
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). After a transfusion, you may need to stay overnight.
FBS and IUT are only available in some hospitals. Therefore, if you need to have one of these procedures, your GP may refer you to a different hospital from the one where you were going to have your baby.
Read more about how rhesus disease is treated.
Diagnosis in a newborn baby
If you are RhD negative, after the birth of your baby blood will be taken from the baby’s umbilical cord to check the blood group and to see whether there is any evidence that anti-D antibodies have crossed the placenta. 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.
Foetal DNA tests
If you are RhD negative and have been sensitised to RhD-positive blood, you are at risk of having a pregnancy affected by rhesus disease if the baby is RhD positive. This will be determined by the father’s blood group.
In some cases, there is only a 50% chance that the unborn baby will be RhD positive, and at risk of rhesus disease. In the other 50% of cases, the baby is RhD negative, like their mother, and safe from the antibodies.
It is now possible to test the rhesus blood group of the unborn baby by taking a simple blood test during pregnancy. Genetic information (DNA) from the unborn baby can be found in their mother's blood and this allows the blood group of the unborn baby to be determined without any risk. It is usually possible to obtain a reliable result from this test by 11 to 12 weeks of gestation, long before the baby is at risk from the antibodies.
If the unborn baby is RhD negative, you can be reassured that your baby is not at risk from the antibodies.
In the future, non-sensitised RhD-negative women may be offered this test to see if they are carrying a RhD-positive or RhD-negative baby. Women carrying a RhD-negative baby will not need to be given anti-D immunoglobulin for prevention.
Read more about how rhesus disease is prevented.
