You may be aware that there are four main blood groups: A, B, AB and O but alongside your blood group there is also the Rhesus factor (Rh). If you have this factor you are described as being Rhesus positive (Rh+ve) and if you do not you are Rhesus negative (Rh-ve).The majority of people are Rh+ve but 16
people out of every 100 are Rhesus negative. For most people, especially men, being Rh-ve is only really of concern if they need a blood transfusion. However, this is different for women, as being Rhesus negative can have an effect on your baby’s health during the first or any future pregnancies. In pregnancy, it is unusual for the mother’s and the baby’s blood to come into contact and mix with each other, as they both have separate circulation systems. For a mother who is Rh-ve and where her unborn baby is Rh+ve, if there is mixing between the two bloods (feto-maternal transfusion), the baby’s positive blood is seen by the mother’s body as a foreign substance (an antigen) to be attacked and destroyed. This sets off a reaction in the mother’s blood to protect her body from what is seen as something that is harmful.The time of greatest risk when this mixing of the bloods can happen is as the afterbirth (placenta) separates or at any time in pregnancy where the placenta is damaged, as this is where the fetal and maternal blood systems are closest. The reaction of the Rhesus negative mother’s blood to the foreign Rhesus positive antigen is called isoimmunisation.This follows the same process as the one used to immunise or protect us from harmful viruses, like polio.The body forms antibodies to fight and destroy the virus therefore preventing us from getting the disease. Antibodies are cells specifically designed to combat the foreign antigen and the presence of these antibodies means that the mother’s body will be sensitised to this particular Rhesus antigen for the rest of her life.These antibodies will then try to destroy the blood of a Rhesus positive baby. This can result in the death of the baby while still in the womb or, if the baby survives, the development of a condition called haemolytic disease of the newborn (HDN), where the baby requires medical care and often a blood transfusion shortly after birth. These antibodies are not usually present in the first pregnancy, as the Rhesus negative mother is unlikely to have had contact with any Rhesus positive antigens before the pregnancy. It is possible to prevent the formation of antibodies by giving the mother immunoglobulin.This blocks the sensitisation process and returns the maternal blood system back to how it was at the start of that pregnancy. The immunoglobulin, commonly called anti-D, is a specially processed, blood-derived product that can only be given by intramuscular injection. This can be given antenatally (ie prophylactically), where there is a risk of contact between maternal and fetal blood and sensitisation could occur. It may also be given as a treatment (therapeutically), where it is highly likely that fetal and maternal blood have come into contact with each other, which would happen most often after the birth of the baby. If you choose to have the injection of anti-D you can be reassured that there is much less risk of haemolytic disease of the newborn (HDN) affecting the baby in your current pregnancy or a baby in a subsequent pregnancy.There are, however, a few issues that you need to know about so that you can make an informed choice. If both the father and the mother of the unborn baby are Rhesus negative, they can only have a Rhesus negative baby and the mother will not require anti-D. Likewise, if the blood type of the baby is known to be Rhesus negative, either from tests done in pregnancy or soon after the birth, these mothers do not need anti-D.
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