Why is having the Rhesus blood test so important during pregnancy?

In order to understand why the Rhesus blood test is an important part of the antenatal screening process, we need to understand what this protein is.

The Rhesus factor is a protein that is inherited and is located on the surface of red blood cells. One is regarded as Rhesus (Rh) positive if the protein is present on these cells and Rh negative if it is absent.

Rh positive is the more common blood type of the two and having this protein is not considered a condition and does not affect the health status of an individual. It may, however, affect a pregnancy if the expectant mother is Rh negative and the father of the baby is Rh positive. In this scenario, the fetus may be either Rh positive or negative but if it takes on the father’s blood compatibility this will cause the mother’s immune system to react to the presence of the Rh factor and produce Rh antibodies that will attack the protein on the red blood cells of the baby.

When is being Rh negative a problem?

Although the mother and the baby’s blood do not mix during fetal development, the fluids may come into contact with each other at certain times of the pregnancy and during delivery of the baby. When this happens the antibodies will be produced but if this is the mother’s first pregnancy and delivery of a baby then the antibodies will not be in contact with the red blood cells of this child.1

However, if the same mother becomes pregnant again with the same father or another Rh positive partner, the already present and circulating Rh antibodies will have plenty of time to attack the Rh proteins on the red blood cells of this fetus.

What do Rh antibodies to the baby’s red blood cells?

The Rh antibodies attack the Rh protein on the red blood cells causing inflammation and damage to the membranes of these cells which leads to damage and rupture of the membrane. This is known as hemolysis.

The hemoglobin protein, which is responsible for oxygen transport to the tissues and organs of the body, leaks out of the red blood cells and this result in anemia which can be life-threatening to the developing fetus. The condition is referred to as hemolytic anemia of a newborn.

What tests are done to check for Rh compatibility?

An initial Rh test will be done during the expectant mother’s first antenatal consultation. If the result is Rh positive then there are no further tests to be done regarding this protein even if the father of the baby is Rh negative. If the fetus is Rh positive this is compatible to the mother and if Rh negative will only form antibodies that will affect the mother’s red blood cells and, as mentioned, the exposure time between the two individuals’ blood will not be long enough to cause any complications.

If the mother is Rh negative, the father will also be tested and if he is Rh positive another test will be performed at 28 weeks of the pregnancy to determine whether Rh antibodies are present in the mother’s bloodstream or not.

What measures are taken if the mother is Rh negative and father is Rh positive?  

In this case, if there are no Rh antibodies produced by 28 weeks of the pregnancy the mother is injected with Rh immunoglobulin (RhIg) which prevents the immune system from producing the antibodies.2

If the baby is born Rh negative then no further management is necessary but if the baby is Rh positive then another dose of RhIg will be administered to the mother shortly after delivery.

If the Rh antibodies are already present in the mother’s bloodstream then RhIg will not help. The baby will be monitored during the pregnancy and if necessary a blood transfusion will be delivered via the umbilical cord or immediately after delivery.2