Rhesus Factor and Anti-D Prophylaxis

One of the blood tests offered routinely at pregnancy booking is to your check blood group. This will also discover whether you are rhesus negative or positive and if you are negative whether or not you have any antibodies that may adversely affect your baby.

Doctor takes blood on the analysisYour blood type is not quite as simple as the O, A, B and AB groups and rhesus factor – although these are the major indicators of blood type, there are a number of other factors within your blood that determine if a sample is an ideal match or not for you, should you need a transfusion. Although a basic match of blood can be administered to you in an emergency, a transfusion that is as close to your specific blood typing is desirable to avoid any transfusion reactions. These lesser known factors are analysed within the lab and kept on your hospital record. Most laboratories like to have two samples from you during pregnancy, so that they can check them against each other and ensure that they are both identical and therefore from the same person (you!). This limits any potential error in matching a sample should you need a transfusion. This is why some hospitals do a further test at 28 weeks gestation, in addition to the booking screen.

Besides being useful if a transfusion is needed, a blood typing that indicates you are rhesus negative means that you can be counselled and offered routine antenatal anti-d prophylaxis (also known as RAADP). If you are rhesus negative it means that you do not have the rhesus factor in your blood in comprison to Rhesus positive people do have the rhesus factor in their blood. As your baby inherits their blood typing from the biological mother and father there is a chance that your baby will also be a rhesus positive blood type. For most pregnancies this does not cause a problem as the mother’s blood system and baby’s blood system are separate. However, occasionally there may be a ‘potentially sensitising event’ where the mother’s blood potentially comes into contact with the baby’s blood. This could be an obvious sensitising event – a blow to the abdomen, a car accident, vaginal bleeding, an invasive antenatal screening test such as amniocentesis, or even during the birth itself. Occasionally however, a potentially sensitising event might not be so obvious.

For a mother who is rhesus negative, the body does not recognise the rhesus factor and reacts against it as a “foreign” object, producing antibodies that attack the rhesus factor. These antibodies can cross the placenta and will effectively attack the baby’s blood too, potentially causing the baby to become anaemic whilst still in utero. The effect of the antibodies can persist for a few months after the baby has been born, which can cause further symptoms of anaemia, poor feeding, increased breathing rate and a prolonged, more severe jaundice than the ‘normal’ physiological jaundice experienced by many healthy newborns.

The first time that the body encounters the rhesus factor it usually produces a weak response against it. However, with repeated exposure to the rhesus factor, the immune system mounts stronger and stronger responses, releasing larger amounts of antibodies to attack the ‘foreign’ rhesus factor. Hence, a first born baby may have very few symptoms or side effects from rhesus disease, but subsequent rhesus positive babies tend to be more and more severely affected.

Administration of RAADP to pregnant rhesus negative mothers has been shown to reduce the incidence of rhesus disease in newborns, and so it is offered to all rhesus negative women at 28 weeks. Additional doses of Anti-d (the same injection given as is given for RAADP) may be needed if a sensitising event occurs, or following the birth. Mothers who are rhesus negative will be offered a final screening once the baby is born. A sample of blood will be taken from the mother and a further sample will be taken from the baby’s umbilical cord to determine their blood type. If the baby is also a rhesus negative blood type then no further anti – d prophylaxis will be offered. If the baby is rhesus positive, however, then a final dose of anti-d will be offered before the baby is 72 hours of age.


What is in the Anti-D Injection?

Anti-D is given as an injection and is made from blood plasma collected from blood donors. It works by destroying any rhesus positive blood from the baby present in the mother’s circulation before she can make her own antibodies, hence why it is important that it is administered promptly, within 72 hours, of any suspected potentially sensitising event. As it is a blood product, it is subject to screening to ensure that there is very little chance of contracting a known virus from having the anti-d injection (it has been estimated that the chance of viral transmission is 1 in 10,000 million doses – i.e. 1 in 10,000,000,000 doses). Like any medicine however there is always the potential for adverse reactions and side effects. You may wish to discuss these further with your Midwife or obstetrician. Additionally, if both biological parents are rhesus negative (in this instance the baby will inherit a rhesus negative blood group), or if you are not planning on any future pregnancies, you may like to talk over your choices with your health professional.

By Lorraine Berry – Midwife

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