Rhesus Incompatibility

A mismatch between the Rh blood group of a pregnant woman and that of her fetus

  • Due to incompatibility of genetically determined blood groups
  • Age and lifestyle are not significant factors

One of the systems for classifying blood is the Rhesus (Rh) group. This system classifies blood according to the presence or absence of certain proteins on the surface of red blood cells. About 17 in 20 people in the UK have Rh proteins on the surface of their red blood cells and are Rh positive. The remaining 3 in 20 people do not have these proteins and are therefore Rh negative.

Rh incompatibility occurs when a mother is Rh negative and her fetus is Rh positive. The situation rarely causes problems in a first pregnancy. However, the mother may develop antibodies if stray blood cells from the baby enter her circulation. These antibodies may attack red blood cells in an Rh-positive fetus in a future pregnancy. The destruction of fetal blood cells may result in severe anaemia in the fetus and in anaemia and yellowing of the skin and whites of the eyes (see Neonatal jaundice) in the newborn baby.

Women who are Rh negative and have no antibodies are now routinely given preventive treatment at 28 and 34 weeks of pregnancy and soon after the delivery, and fewer than 1 in 100 women develop problems in future pregnancies.

What is the cause?

Blood groups are inherited from both parents. A baby who is Rh positive can be born to an Rh-negative mother only if the baby’s father is Rh positive. The circulatory systems of the mother and the fetus are separate, and the red blood cells do not usually cross from one to the other. However, there are circumstances in which stray red blood cells from the fetus can enter the mother’s circulation. The fetus’s blood cells may leak into the mother’s system during delivery, miscarriage, or termination of pregnancy. There is also a risk of blood mixing when an amniocentesis test is carried out (see Antenatal genetic tests) or after a placental abruption, in which part or all of the placenta detaches from the uterus before delivery. The mother’s immune system reacts by producing antibodies to destroy the fetal red blood cells in her circulation. In future pregnancies in which the fetus is Rh positive, these antibodies cross the placenta and destroy fetal red blood cells. Untreated, these effects become increasingly severe in each subsequent Rh-incompatible pregnancy.

What are the effects?

The mother remains well and is usually unaware that there is a problem. The effects on the fetus depend on the level of antibodies present and when in the pregnancy they are produced.

The fetus may develop swelling and progressive anaemia, in which destruction of the red blood cells leads to low levels of oxygen-carrying pigment in the blood. Rarely, a severely anaemic fetus develops acute heart failure and may die in the uterus (see Stillbirth). After an Rh-incompatible pregnancy, a baby may be born with severe anaemia. Jaundice in the newborn baby occurs due to build-up of bilirubin, a pigment produced from the destruction of fetal red blood cells. Rarely, severe jaundice may cause brain damage.

What might be done?

If Rh antibodies develop, treatment depends on the amount of antibodies and their effect on the fetus. A sample of the fluid in the uterus is tested for evidence of high bilirubin in the fetus. A sample of fetal blood may also be taken from the umbilical cord and tested for haemolysis (destroyed red blood cells). Additional ultrasound scanning (see Ultrasound scanning in pregnancy) may be used to check whether the fetus is swollen. If antibody levels are low, the pregnancy may continue until labour is induced at 38 weeks (see Induction of labour); if levels are high, labour may be induced earlier. A fetus that is too immature for delivery may have a blood transfusion of Rh-negative blood into the umbilical cord or abdominal cavity. After birth, the baby may need more transfusions and treatment for jaundice.

Can problems be prevented?

All women are tested at their first antenatal visit to determine their Rh blood group. If you are Rh negative, you will have a blood test at about 28 weeks to see if you have developed antibodies. You will also be given an injection of antibodies against Rh-positive blood at about weeks 28 and 34 and soon after the birth to destroy any fetal red blood cells in your blood. This prevents you from developing antibodies that might react against future Rh-positive fetuses. You may also have this injection after a miscarriage or other procedure that causes fetal and maternal blood to mix. With this treatment, you are very unlikely to develop antibodies that will cause problems in the future.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

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