Diagnosing Rhesus Disease During Pregnancy

Rhesus Disease is an unwanted complication of pregnancies that occurs because of an incompatibility between maternal and baby blood. Alongside the usual blood types, human blood can be split into two groups based on the presence of a particular substance on red blood cells called the rhesus D antigen. The presence of the rhesus D antigen makes you a rhesus positive person, while the absence would make you rhesus negative.

Rhesus disease occurs when an unborn baby is rhesus positive, but his or her mother is rhesus negative. If maternal blood is exposed to baby blood, it reacts to it, treating it as an invading organism. In rhesus disease, the elements of the maternal system attacking rhesus positive blood is called antibodies make their way into the baby’s body and begin to cause unpleasant symptoms which, if left untreated, can even cause death.

Fortunately modern screening and testing technologies provide us with a means of testing for the rhesus D antigen, which is usually abbreviated to RhD. A blood test is usually conducted early on during the pregnancy to test for a number of conditions that can be dangerous to a pregnancy like HIV and hepatitis B. This test is also used to find out what ABO blood group you fit into (whether you are B+ or AB- etc) and whether or not you are rhesus positive or rhesus negative.

Your blood will be checked again for anti-D antibodies towards the end of your pregnancy at about the 28 week mark. If you don’t have anti-D antibodies, meaning that you haven’t been sensitised, then your doctors will regularly give you a preventative anti-D preparation (called prophylaxis) that stops your body from generating the defending antibodies.

If the blood tests show that you have been sensitised to RhD antigens, then don’t fret, there are steps that can be taken to ensure the health of your pregnancy. You are likely to be monitored more during your pregnancy, particularly using a special kind of ultrasound called the Doppler ultrasound to measure blood flow through your baby’s brain. This is a way of checking whether or not your baby is suffering from one of the effects of rhesus disease, anaemia, as anaemic blood is thinner and flows through the brain more quickly.

If the Doppler test shows that your baby might be anaemic, a further test called FBS, or foetal blood sampling, is performed to check for anaemia. A needle is used for this, and passed through the abdomen and into the womb to extract the small amount of blood needed for the test.

Amniocentesis can also be potentially used to test for rhesus disease. This disease is in principal similar to FBS, but rather than blood, a special fluid encasing the unborn baby called the amniotic fluid is extracted and tested. This test relies on a genetic test of your child’s cells to find out whether or not your child is rhesus positive or negative. That being said, the FBS test is much safer and so used more often for the purposes of checking for rhesus disease.

Newborn babies can be tested for the presence of rhesus disease if you are rhesus negative by means of a test of the umbilical cord, which will bear evidence of any anti-D antibodies that may have been transferred from the mother across the placenta.

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