Important: This is intended as general information only. It may not apply to your specific situation.  It is not a substitute for medical advice.  If you have a question or concern, please raise it with me at your next appointment or contact me sooner if it's urgent.

Blood group and pregnancy

Everyone has a ‘blood type’ which is determined by the presence or absence of various proteins on the surface of their red blood cells.  These proteins don’t have any effect on how well your red blood cells do their job - which is to carry oxygen around the body.  However, they do determine who you can receive a blood transfusion from, and they can cause problems in pregnancy if there is a ‘mismatch’ between the blood group of the mother and the baby.  The particular proteins each person has on their red blood cells is genetically determined.

The following information is quite complicated – don’t worry if it doesn’t completely make sense to you.  I’ll discuss your blood group with you at your first antenatal visit and I’ll make sure that you understand anything that is relevant to your situation.

There are lots of different ways to classify red blood cells but the two most important systems are the ABO and rhesus systems.  We’ll deal with ABO system first.

ABO blood group system

People are classified as blood group A, B, AB or O depending on whether they have two proteins (known as the A antigen and B antigen) on the surface of their red blood cells.  People with blood group A have the A antigen and people with blood group B have the B antigen.  People with blood group AB have both and people with blood group O have neither.    Generally speaking, your immune system will react against anything that is not part of your body.  In simple terms, this means that the immune system will produce antibodies to attack any ‘foreign’ proteins it detects.   This is why some blood types are incompatible.  If you have blood group A, your immune system will mount a reaction if it encounters blood group B.  If you have blood group B, your immune system will mount a reaction if it encounters blood group A.  People with blood group O will react to both A and B blood types and people with AB don’t react to either.  Choosing an appropriate ABO type is the first step in preparing a transfusion.  Of course, in real life, the actual cross-matching process is much more complicated than this.

It’s possible for a baby to have a ABO type that is incompatible with his or her mother’s.  Even when the blood types are technically incompatible, most of the time this doesn’t cause any problems.  When it does, the problems are usually mild and happen after the baby is born, which makes them much easier to deal with. 

Rhesus blood group system

Our main concern in pregnancy is with the Rhesus (or Rh) system.  People are classified as Rh positive or negative, depending on whether they have the Rh protein on the outside of their red blood cells.  As you might imagine, people who have the protein are Rh positive and people who don’t have the protein are Rh negative.   If someone with Rh negative blood encounters red blood cells from someone who is Rh positive, their immune system will recognize the Rh protein as ‘foreign’ and mount a response against the red blood cells carrying this protein. Once a person has developed antibodies, they are present for life.

This can be a problem in pregnancy for Rh negative women who are pregnant with a baby that is Rh positive.  In contrast to antibodies against the A and B antigens, antibodies against the Rh protein do cross the placenta and can cause a severe immune response leading to breakdown of the baby’s red blood cells.

How these antibodies develop

Anti-Rh antibodies can develop if at any stage if a person with Rh negative blood is exposed to Rh positive blood.  This might occur in an incompatible transfusion, during childbirth, a miscarriage, a termination of pregnancy, some pregnancy procedures (for example amniocentesis) or trauma during pregnancy.  Even without any of these events, a small number of women can develop antibodies due to the fact that a small amount of the baby’s blood always manages to sneak across the placenta and into the mother’s circulation.  Fortunately, providing we know that a woman is Rh negative, we can take steps to reduce the risk that she will develop antibodies in all of these situations.

How to prevent the development of antibodies  

If we think that a Rh negative woman is at risk of developing antibodies we can give her an injection of Rh antibodies that have been extracted from the blood of blood donors – known as prophylactic anti-D.  Unlike a person’s own antibodies, these antibodies don’t last for ever.  They remain in the woman’s circulation for about 6 weeks, during which time they go around and ‘mop up’ any Rh proteins so that the woman’s own immune system doesn’t see them and form it’s own antibodies.  We vary the dose depending on the gestation and the ‘event’ that has occurred.  Therefore, if a Rh negative women has a miscarriage, a termination, a procedure during pregnancy, trauma during pregnancy, or any bleeding during pregnancy we would give her an injection of anti-D. 

A small number of women will develop antibodies during their pregnancy even if none of the events above happen.  To try to prevent this, we give two doses of prophylactic anti-D to all Rh negative women during the pregnancy – at about 28 weeks gestation and again at about 34 weeks gestation. 

During the pregnancy, we’re operating on the assumption that the baby is Rh positive – a reasonable assumption since most people are Rh positive.  Once the baby is born, we can test his or her blood group by taking some blood from the cord.  If the baby is, in fact, Rh positive, we give the mother a further dose of anti-D in the first couple of days after the birth.  However, if the baby is found to be Rh negative, this postnatal dose is not required.

Giving prophylactic anti-D works really well and significantly decreases the likelihood that Rh negative women will develop antibodies.  However, like all treatments, is does not work in 100% of cases.  A very small number of women will develop antibodies even if anti-D is given appropriately. 

What happens if Rh antibodies develop

In most cases, if Rh antibodies develop during pregnancy, they aren’t a major problem in that pregnancy.  The problems usually arise in subsequent pregnancies.  The antibodies can cross the placenta and attack the baby’s red blood cells.  As the baby’s number of red blood cells falls, it becomes progressively harder for oxygen to move around the baby’s body.  Initially, the baby can compensate for this by increasing it’s heart rate and directing blood to the most important parts of the body.  In severe cases, the baby can become seriously unwell and this condition can be fatal.

If you are found to have antibodies before or during your pregnancy, I will arrange for very close monitoring of you and your baby.  This is done mostly through blood tests to measure the antibody levels and ultrasound monitoring looking for signs of fetal anaemia.  There is treatment available both during the pregnancy and for the baby after birth.  The most important thing is that we know that the antibodies are present so we can monitor the baby closely.  For this reason, all women who are Rh negative will have blood tests during the pregnancy to monitor their antibody status.

Other red blood cell antibodies

There are many other proteins that can be present on the surface of red blood cells.  It’s possible to develop antibodies against these other proteins although this happens much less commonly than antibodies against the A, B or Rh proteins.  Therefore, all pregnant women should have their blood tested for antibodies at the beginning of and during the pregnancy.  I will arrange this as part of your routine pregnancy testing.