Explaining the Rhesus factor
IF you have ever done a blood test, you would have found out whether you are from blood groups A, B, AB or O. You would notice too that your blood type also has either a positive or negative symbol next to it (for example, A+ or O-). The plus or minus sign indicates whether the rhesus D (RhD) antigen is present in your blood.
While this does not affect growth or development in most people, it is important for a pregnant woman to know whether she is RhD positive or negative, as it can affect how her body reacts to her child.
Consultant obstetrician-gynaecologist Dr Tiffany Hunter explains that Rhesus disease occurs when an RhD negative mother is carrying an RhD positive child, and her body develops an immune reaction to the foetus.
“But not all RhD negative mothers carrying an RhD positive child have rhesus disease,” she clarified. “It only happens when the mother’s body is producing antibodies that are attacking the unborn child. When this happens, she is said to be isoimmunised or sensitised to her baby’s RhD blood, and her body recognises it as foreign.”
She points out that a woman’s body will only produce these antibodies after she has been exposed to RhD blood at least once before, so usually she will not have a reaction to her first RhD positive child. But during the first pregnancy, low levels of foetal blood cells might cross into the mother’s bloodstream, or she might be exposed to the baby’s blood during delivery, or if there is any bleeding during the pregnancy. If she becomes pregnant with another RhD positive child, her body, by that time, may become very resistant to the blood.
Dr Hunter explains the procedure when a mother is found to be RhD negative.
“We always check to see if she has antibodies,” she says. “If she does have those antibodies, then we check to see how concentrated those antibodies are. This indicates the seriousness of the reaction, and we monitor it to see if it is worsening.
“We also do ultrasounds on the mother, looking at the blood flow to the baby’s brain, which will give an indication if the baby has anaemia. If the baby does we normally consider either delivering the baby or performing a blood transfusion. We would choose the option in which the benefits outweigh the risks, and also depending on what is available at the time.”
Dr Hunter says that undetected or untreated rhesus disease can have dire effects on the foetus.
“The mother’s body can create antibodies that fight against the baby’s blood, destroy the baby’s red blood cells, and cause the baby to be anaemic. If this anaemia goes undetected, the baby can develop fluid around some of the organs such as the heart, the lungs, and within the abdomen. This is called immune hydrops, and can cause the foetus to go into heart failure and die while in the mother’s womb,” she says.
Luckily, though, the worst effects of Rhesus disease can be avoided. Dr Hunter explains that blood tests are done on babies even after a safe delivery, and if they are found to be RhD positive and born to an RhD negative mother, the mother will be given a medication called RhoGAM, which prevents her from becoming isoimmunised.
“Sometimes though, for varying reasons, a mother may not be given this medicine, and her body will become immunised. Because of this, it important for blood tests to be carried out everytime a woman becomes pregnant,” she says.
