Rhesus-negative blood and pregnancy

by | Babies and Pregnancy, Heart Attacks and Strokes

If you have rhesus negative (RhD-) blood and are pregnant, your fetus may be at risk of health problems caused by rhesus disease. Rhesus disease is an incompatibility between the blood types of the mother and the baby. It happens when a rhesus negative woman carries a fetus with rhesus positive (RhD+) blood.

Fortunately, even though your baby’s blood type is not usually known until delivery, a simple treatment during pregnancy can help prevent problems in women who are RhD negative.

What is rhesus (Rh) factor?

Rhesus factor is an inherited protein called RhD which is found on the surface of red blood cells. Rhesus factor was originally named (incorrectly) after rhesus monkeys, but now scientists more correctly refer to it as Rh factor.

Not everyone’s blood is the same. A person’s blood is either Rh positive (has the RhD protein) or Rh negative (doesn’t have the RhD protein). Most people have the Rh factor and test Rh positive (RhD+), however, some people don’t have the Rh factor and so are Rh negative (RhD-). In Australia, around 80 per cent of people are Rh positive.

Blood is classified into groups, the most well-known being the ABO system in which a person’s blood is recorded as either A, B, AB or O. Each blood type is also further identified by a plus (+) or minus (-) sign, which designates the Rh status of the blood. For example, someone’s blood may be ‘O positive’ (written O+) and another person may be ‘AB negative’ (written AB-).

Blood transfusions and blood groups

When someone needs a blood transfusion it is preferable that they are transfused with blood from the same ABO and RhD groups as their own. Otherwise a reaction to the ‘foreign’ blood may occur. If this is not available, they may be given blood from a compatible blood group.

Most people are Rh positive (RhD+). But if an Rh negative (RhD-) person receives Rh positive (RhD+) blood, their body reacts, making chemicals (antibodies) to defend against the foreign Rh factor. This can cause a transfusion reaction. Mild transfusion reactions are not dangerous, but a severe transfusion reaction may be deadly.

Why is Rh factor important in pregnancy?

On its own the Rh factor does not cause health problems. But when a mother is Rh negative and her fetus is Rh positive, the mother’s blood may produce antibodies against the baby’s Rh factor. These antibodies then attack the unborn baby’s red blood cells. This can cause health risks for that fetus and for the fetus of any subsequent pregnancies she may have.

A pregnant woman who is Rh negative, who has a fetus which is Rh negative won’t have any problems.

A pregnant woman who is Rh positive won’t have any problems (with antibodies to RhD) , regardless of whether the baby is Rh positive or negative.

How do you know if you are Rh negative?

A simple blood test can show if you are Rh negative. The blood test is done routinely very early on in pregnancy (at the first antenatal visit) and shows your blood type (A, B, AB, or O) and whether you are Rh positive or negative.

If you are Rh negative you may be offered further blood tests later in the pregnancy (at 28 weeks) and at delivery to make sure Rh antibodies have not formed in your blood.

Risks of being an Rh negative mother with an Rh positive fetus

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. This is normal and for most women not a problem. However, an Rh negative mother will develop antibodies to attack the Rh positive blood. This doesn’t often cause problems during a first pregnancy, because there’s usually no significant contact between the baby’s and mother’s blood until the baby is born.

But it 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.

These antibodies will then bind with the baby’s Rh positive red blood cells, causing them to be destroyed. As a result of this, the baby may be born seriously ill, and unless a blood transfusion is given shortly after birth, the baby could die.

In each subsequent pregnancy the mother becomes more sensitised to Rh positive blood, having a stronger immune response which produces antibodies earlier and earlier in each pregnancy. This may mean the baby has anaemia or becomes brain damaged or even dies before it can be born. Antibodies to RhD are a cause of haemolytic disease of the fetus and newborn (HDFN).

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.

1. If the mother is Rh negative and the father is Rh positive, their fetus may be Rh positive or Rh negative.

rhesus-negative mother; rhesus-positive father

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.

rhesus-positive fetus

3. Left untreated, the mother’s blood will make antibodies that attack the Rh positive blood of the fetus.

mother's blood makes antibodies against fetus' blood

4. These antibodies can cause health problems for the fetus, such as anaemia or even death.

these antibodies can cause problems for fetus

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.

Anti-D injection

Fortunately, an injection can be given which stops an Rh negative mother producing the antibodies that attack Rh positive blood. This is known as the ‘anti-D injection’ and contains anti-D immunoglobulin. It is effective in nearly all cases.

In Australia, this anti-D injection is offered routinely to all Rh negative women at 28 and 34 weeks’ gestation to prevent early sensitisation.

Anti-D is also given routinely to Rh negative mothers after birth, miscarriage and terminations. Anti-D should prevent RhD antibodies forming, which would affect any further pregnancies the mother has.

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 RhD 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 RhD antibodies have already formed.

If you already have RhD antibodies

If RhD antibodies have already formed (sensitisation), anti-D injections cannot protect the fetus. The antibodies cannot be removed once they have been made. You and the fetus will need special care during pregnancy. Your doctor or obstetrician will explain the details to you.

Sometimes a woman’s RhD antibody levels need to be measured periodically during her pregnancy to anticipate whether the baby might have problems. Depending on your antibody levels, you may require specialist care and your fetus may require transfusions before birth to prevent the baby being anaemic. If your antibody levels are too high, you may need further tests to check the health of the unborn baby. Sometimes the unborn baby needs a blood transfusion soon after birth.

If you are rhesus negative

If you are Rh negative, ask your doctor or obstetrician about treatment with anti-D immunoglobulin. 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.

Prenatal Rh testing for the fetus

There is now a non-invasive test that can identify the Rh status of a fetus. The test involves a blood sample from the mother. During pregnancy, some of the unborn baby’s DNA circulates in the mother’s bloodstream. The test analyses these fragments of fetal DNA to determine the Rh status of the fetus. However, this test will not be used routinely, and is only for certain high-risk pregnancies.

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