Red Cell Antibodies in Pregnancy

What this test is and when it is done

Every pregnancy in the UK includes blood group and antibody screening. NICE guideline NG201 says to offer a blood test to check full blood count, blood group and rhesus D status at the first face-to-face antenatal appointment, and at 28 weeks to offer a blood test to check full blood count, blood group and antibodies, together with anti-D prophylaxis for rhesus D-negative women who are not known to be sensitised.

The British Society for Haematology guideline on red cell antibodies in pregnancy, published in February 2025, states the purpose plainly: the blood group and antibody status "should be tested at booking and at 28 weeks' gestation to identify the ABO and D group and to detect red cell antibodies that have the potential to be clinically significant". Its aim is to "predict the potential for, and where possible, prevent, haemolytic disease of the foetus and newborn".

This BSH guideline replaced the older RCOG Green-top Guideline on the same topic, which has been archived. If you find a page citing RCOG Green-top No. 65 as current guidance, it is out of date.

This is not the same as being rhesus negative

Two different things get confused here constantly.

Being RhD negative means your red cells lack the D antigen. On its own that is a blood group, not a problem, and anti-D prophylaxis exists to stop it becoming one.

Having a red cell antibody means your immune system has already made antibodies against an antigen your own red cells do not carry, usually from a previous pregnancy or a transfusion. That is what antibody screening detects, and it is what this page is about.

An RhD-negative woman who has anti-D detected on screening in a later pregnancy needs the laboratory to work out whether that is prophylactic anti-D given to her, or immune anti-D she has produced herself. Those look similar on a screen and mean completely different things, which is why the interpretation belongs with the transfusion laboratory rather than with an internet search.

Which antibodies matter

BSH divides them into three practical groups. "Some antibodies (including anti-D, anti-K and anti-c) are associated with significant foetal and neonatal risks, such as anaemia, jaundice and perinatal loss." Then there "are antibodies that are unlikely to significantly affect the foetus but that can cause neonatal anaemia and hyperbilirubinaemia". And there are "others that may cause problems for the screening and timely provision of appropriate blood for the woman or baby".

That third group is easy to miss and worth understanding, because it is the reason a clinically harmless antibody still gets recorded and still matters. Some antibodies make cross-matching blood slower. If you needed an urgent transfusion, the laboratory would need more time to find compatible units, so the antibody is flagged in advance so that blood can be prepared.

Anti-K deserves a specific mention. Unlike most antibodies, which cause fetal anaemia by destroying red cells, anti-K also suppresses red cell production in the fetal bone marrow, so the usual relationship between antibody level and severity is less reliable. That is why anti-K is monitored more closely than its titre alone might suggest.

What monitoring involves

If a clinically significant antibody is found, care moves to a specialist fetal medicine service and usually involves three strands.

• Quantification or titration. Anti-D and anti-c are usually quantified in international units per millilitre in the UK, while other antibodies are titrated. Levels are repeated at intervals through pregnancy, more often as pregnancy advances, so the trend is as important as any single figure.
• Working out whether the baby carries the antigen. If your partner does not carry the relevant antigen, the baby cannot be affected. Where paternal status is uncertain or heterozygous, fetal genotyping from maternal blood is available for several antigens and avoids invasive testing.
• Watching for fetal anaemia. ACOG's Practice Bulletin No. 192 on alloimmunization describes how "advances in Doppler ultrasonography have led to the development of noninvasive methods of management of alloimmunization in pregnant women", allowing "a more thorough and less invasive workup with fewer risks to the mother and fetus". In practice that means serial middle cerebral artery peak systolic velocity measurements rather than repeated amniocentesis.

If the baby does become anaemic

The treatment is intrauterine transfusion, given into the umbilical vein under ultrasound guidance in a specialist fetal medicine centre, repeated as needed until the baby is mature enough to be born safely. This is a well-established procedure with good outcomes in experienced hands, and its existence is the reason the monitoring above is worth doing.

Where anaemia does not develop, the pregnancy usually continues to term with monitoring only. ACOG's bulletin explains the underlying mechanism: antibodies cross the placenta and, "depending on the degree of antigenicity and the amount and type of antibodies involved", may cause haemolytic disease of the fetus and newborn. Many antibodies never reach a level that does.

After the birth

Your baby's cord blood is tested for blood group, haemoglobin and bilirubin, and babies of women with significant antibodies are watched for jaundice and anaemia, which can develop or worsen over the first days and sometimes weeks. Phototherapy and, occasionally, transfusion are the treatments.

For your own future, the antibody does not go away. It is permanent information that affects any future pregnancy and any future transfusion. Ask for a copy of the result and keep it, and mention it at the booking appointment of any subsequent pregnancy rather than assuming it has carried across.

NICE guideline NG121 sets the general expectation that intrapartum care is planned in advance where an existing condition is present, and the NHS screening publication describing tests offered in pregnancy explains where antibody screening sits among them.