Rhesus Negative Test

Your Baby's Blood Group and Anti D

In all pregnancies the mother’s blood group is checked. Should the mother be Rhesus Negative (15% chance in white Europeans) and the father be Rhesus Positive (85% chance in white Europeans) then the baby has a 66% chance of being Rhesus Positive and 33% chance of being Rhesus Negative. If the mother is Rhesus Negative and the baby is Rhesus positive, then there is a small chance that baby’s blood cells can cross to the mother’s circulation and set up an antibody response which can cross back and damage the baby. This can be prevented by giving all mothers an injection of Anti D immunoglobulin. This is manufactured safely from human blood products. However, in 33% of cases the injection is unnecessary.

The routine injection for all mothers has been necessary because until recently it was difficult to know the baby’s blood group when still in the womb. The technology which gives us NIPT (Non-Invasive Prenatal Testing) for Down’s syndrome is also used, by a different laboratory, for NIPT to determine the baby’s blood group. This is done by taking a simple blood sample from a vein in the mother’s arm and sending it to a specialised laboratory. Within a few days the mother will be advised of the baby’s blood group. Should it be Rhesus Negative then nothing more needs done, and you do not require the Anti D injections. Should the baby be Rhesus Positive you will need to discuss the need for an Anti D injection with a midwife. The NHS accept these results. In fact, the test is done in an NHS laboratory although it is generally not available in NHS care.

How and when will I receive my results?

The blood sample is processed by an NHS laboratory and the results take between 10 and 14 days. We will email the results to you. This laboratory report should then be added to your primary care notes e.g your NHS pregnancy records.

What happens if the result is D negative?

If your unborn baby is predicted to be D negative, please discuss the results with your primary care midwife who will recommend that you do not have anti-D injections before or after giving birth. However, there is a very small chance (0.1%) that the baby might be found to be D positive once born. In this case you are likely to be offered an anti-D injection at that time by your NHS Hospital.

What happens if the result is D positive?

The Rhesus blood test is predictive, with a positive predictive value of 98%. This means that we would expect 98 out of 100 babies with a predictive positive result to be confirmed as D positive after birth, however, 2.0% (2 out of 100 babies) may in fact be D negative. 

If your blood test report predicts that your unborn baby is D positive, or the result is inconclusive, your primary care team will offer an anti-D injection. This is of no concern as anti-D prophylaxis would have been offered in all cases if DNA testing had not taken place and the injection will not harm your baby.

For more information: Rhesus disease - Prevention

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