Rhesus Factor: Effects On Pregnancy

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What is Rhesus factor? – Rhesus (Rh) factor is an inherited protein (antigen) found on the surface of red blood cells. When the antigen is present, it is assessed as being positive but negative when it is not present. Thus blood groups are represented as A+ / A-; B+ / B-; AB+ / AB+ and O+ / O-, with the + sign indicating the presence of Rh antigen while the – sign indicates its absence.

Knowing the Rh status of an individual is important for blood transfusion and for management of women in pregnancy. There are many Rh antigens but the most important of these, which is also tested for, is Rh D antigen.

How common is Rhesus antigen?

Majority of people (about 85%) have the Rhesus antigen; thus they are Rh positive while an estimated 15% of the population is Rh negative. 

What is the implication of being Rhesus positive or Rhesus negative?

People with Rh antigen have no problem receiving blood from those without Rh antigen. However, those without Rh antigen produce antibody against Rh positive red blood cells if they are transfused with such blood. This antibody attacks and destroys such Rh positive red blood cells. Therefore, except in dare emergency situation, people who are Rh negative should not be given Rh positive blood type.

What are Sensitising Events?

Sensitizing events are those activities that allow Rh positive red blood cells to enter into the blood circulation of someone who is Rh negative. These include blood transfusion, pregnancy and childbirth, bleeding in pregnancy, miscarriage and certain manoeuvres in pregnancy etc. These Rh positive red blood cells stimulate the production of anti-Rh antibody in Rh negative individual.

This antibody attacks and destroys Rh positive red blood cells in order to eliminate them from the blood circulation. This process of antibody production is known as sensitisation while the effect on the baby, caused by the antibody, is referred to as Rhesus isoimmunisation.

What is the implication of Rhesus factor for pregnant women?

In pregnancy, even though the mother’s and fetal blood flow in different compartments without mixing, there is always a little chance of fetal blood finding its way into the mother’s blood circulation. When the mother is Rh positive, this is of no consequence irrespective of the Rh status of the fetus. However, when the mother is Rh negative, the Rh status of the fetus becomes important.

If the fetus is Rh negative like the mother, the fetal blood getting into maternal circulation is of no consequence. However, when the fetus is Rh positive while the mother is Rh negative, the fetal red blood cells that find their way into the mother’s blood circulation stimulate the mother’s immune system to produce antibody targeted at the Rh positive red blood cells to eliminate it. 

How does the antibody to Rhesus antigen work?

When this antibody is released into the mother’s blood circulation, it destroys the Rh positive red blood cells in the mother’s blood circulation. This is the primary purpose of producing the antibody. In addition to this, however, it crosses the placenta into the blood circulation of the fetus and destroys the red blood cells with Rh positive in the fetus.

Luckily, the process of producing this antibody takes a reasonable length of time. This is because it has to be specific to the targeted agent that needs to be eliminated. As a result of the delay in producing the antibody, the first pregnancy, which sets up the process of antibody production in the mother, is almost always unaffected. This is because, by the time the antibody will be ready to work, delivery of the first child would have taken place.

However, the mother’s body preserves the memory of this inciting agent (Rh positive red blood cells) for future purposes. So, when it encounters such in the future (subsequent pregnancies with Rh positive fetuses), it quickly produces enough antibodies within a short time to attack and destroy the Rh positive red blood cells anywhere they are found, including the fetus.

As a result of this, the subsequent pregnancies are easily affected by the anti- Rh antibody in the mother. The red blood cells in the fetus are attacked and destroyed, making the fetus to lack adequate red blood cells (anaemia). 

The extent to which the fetus is affected depends on the quantity of the antibody in the mother. When the antibody level is minimal, the red blood cell destruction will be minimal and baby can be monitored till term or close to term before delivery. After delivery, interventions like exchange blood transfusion may be required to make the baby healthy.

On the other hand, when the antibody level is very high, it may result in fetal demise if action is not taken to treat the fetus before delivery. Affected babies may appear bloated with generalised body oedema at delivery. 

What Interventions Are Required For Affected Fetuses?

Affected fetuses of sensitised mothers are monitored during pregnancy using various methods such as frequent fetal blood sampling or Doppler ultrasound. Interventions are indicated when a fetus shows evidence of severe anaemia in pregnancy or after delivery. Some of the interventions include:

  1. In-utero blood transfusion (Blood transfusion in pregnancy) – The fetus is transfused with blood when the risk of anaemia to the fetus becomes very high during pregnancy. This is to prevent complications such as heart failure in the fetus, as the heart overworks when there is severe anaemia.
  2. Exchange blood transfusion – Sometimes, the anaemia is not too severe before delivery as to warrant in-utero blood transfusion. However, after delivery, the by-products of the red blood cell destruction (such as bilirubin) accumulate in the blood of the baby and become too much for the immature liver to cope with. This may lead to jaundice and accompanied complications. As a way of preventing or treating the jaundice, the baby’s blood is removed stepwise and replaced with fresh blood that is Rhesus negative. By doing this, (a) The antibody causing red blood cell destruction is removed, (b) Excess bilirubin in the circulation, which may lead to jaundice with its attendant complications, is removed (c) Rhesus negative red blood cells that cannot be attacked by the antibody are introduced. This measure effectively takes care of the effect of Rhesus antibody on the baby.

How can Rhesus isoimmunisation be prevented?

Rhesus isoimmunisation is better prevented than treated; and there are simple measures by which it can be prevented. These include:

  1. Avoiding transfusion of someone who is Rh negative with Rh positive blood 
  2. Avoiding any procedure that can trigger the escape of fetal blood into the mother’s blood circulation
  3. Administration of anti D immunoglobulin if there is any such procedure that could trigger escape of fetal blood into maternal circulation or miscarriage. This is also administered at about 28 weeks into normal pregnancy in a Rh negative woman and after delivery

These measures help in preventing the red blood cells that escape into the mother’s blood circulation from stimulating the mother’s immune system to produce antibody against Rhesus positive red blood cells. Thus the subsequent pregnancies are spared the effects of mother’s antibody that may attack the fetus in pregnancy. 

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