Rhesus-negative blood and pregnancy
Why be concerned?
If you have rhesus negative (Rh-) blood, your fetus may be at risk of health problems. This is true only if the fetus has rhesus positive (Rh+) blood; however, usually your baby’s blood type is not known until delivery. A simple test followed by treatment can help prevent problems.
What is rhesus factor?
Rhesus factor is a substance found in people's blood. Everybody's blood is either rhesus positive or rhesus negative. Most people have the rhesus factor (Rh) and test rhesus positive (Rh+), however, some people are rhesus negative (Rh-) and don't have the Rh factor.
On its own the rhesus factor does not cause health problems. But when a mother is Rh negative and her fetus is Rh positive, there can be health risks for that fetus and for the fetus of any subsequent pregnancies she may have.
1. If the mother is Rh negative and the father is Rh positive, their fetus may be Rh positive or Rh negative.
2. If the fetus is Rh positive, there is a risk that some of its Rh positive blood cells will get into the mother's bloodstream during the pregnancy or delivery and mix with her Rh negative blood.
3. Left untreated, the mother’s blood will make antibodies that attack the Rh positive blood of the fetus.
4. These antibodies can cause health problems for the fetus, such as anaemia or even death.
5. If the mother then has another baby later on that is also Rh positive, her antibodies may cross the placenta and attack the baby's blood, destroying its blood cells.
What are the risks?
The mother's blood does not normally mix with the baby's blood during the pregnancy, unless there has been a procedure (such as amniocentesis or chorionic villus sampling) or vaginal bleeding. During delivery, however, there is a good chance that some of the baby's blood cells will enter the mother's bloodstream. The mother will then develop antibodies to attack Rh positive blood. This has implications for any further pregnancies the mother has — if she carries another Rh positive baby, her body will produce antibodies which cross the placenta and attack the baby's blood cells. This may mean the baby has anaemia or becomes brain damaged or even dies before it can be born.
The chance of forming Rh antibodies occurs with each pregnancy, including ectopic pregnancies (when the fertilised egg implants itself outside the uterus) and pregnancies that end in miscarriage or termination.
What can be done?
Fortunately, an injection of serum can be given which stops the mother producing the antibodies that attack Rh positive blood. This injection is known as anti-D and contains anti-D gammaglobulin.
How do you know if you are rhesus negative?
A simple blood test can show if you are Rh negative. The test is routinely done very early on in pregnancy. If you are Rh negative you may be offered further blood tests later in the pregnancy to make sure rhesus antibodies have not formed in your blood.
When will you be treated?
Anti-D is given to Rh negative mothers who have given birth to an Rh positive baby. The injection is usually given within a few hours of giving birth. Anti-D should prevent rhesus antibodies forming, which would affect any further pregnancies the mother has.
To prevent early sensitisation, anti-D is also given to Rh negative women at 28 weeks of pregnancy and again at 34 weeks.
Earlier or additional doses of anti-D are also generally given if there is an episode of vaginal bleeding during the pregnancy, and when invasive tests such as amniocentesis or chorionic villus sampling are performed. Women who have a miscarriage, an ectopic pregnancy or a termination of pregnancy will also need anti-D, even if it is the first pregnancy, to protect future pregnancies. These are all situations where fetal blood may mix with the mother's blood causing Rh antibodies to form.
Anti-D should be given within 72 hours of the immune system coming into contact with blood from the fetus. It is too late to give the injection if rhesus antibodies have already formed.
If you have rhesus antibodies
If antibodies have already formed (sensitisation), anti-D gammaglobulin cannot protect the fetus. You and the fetus will need special care during pregnancy. Your doctor or obstetrician will explain the details to you. Depending on the antibody levels, you may require specialist care and your fetus may require transfusions before birth to prevent the baby being anaemic.
If you are rhesus negative
If you are Rh negative, ask your doctor or obstetrician about treatment with anti-D gammaglobulin. Even if you miscarry or do not deliver the baby, you will still need treatment. The health of any baby you have in the future depends on it.
Last Reviewed: 05 August 2009
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