Free Foetal DNA

During pregnancy after 12 weeks of gestation, a small amount of foetal DNA can be found in the maternal circulation. This DNA can be tested to predict the babys Rh(D) status using polymerase chain reaction. Knowing the baby's Rh(D) status is useful in Rh(D) Negative pregnant women who have not formed their own immune anti-D as it determines whether or not they will require anti-D prophylaxis throughout the pregnancy.

A positive test means that the foetus is predicted to be Rh(D) Positive and in the case of twins, at least one foetus is Rh(D) Positive. These patients will require standard anti-D prophylaxis. A negative test means the foetus is predicted to be Rh(D) negative and anti-D prophylaxis is not required. A cord sample at delivery should still be tested in the negative test patients to confirm.

Sampling for ffDNA testing follows the same sampling guidelines as other transfusion samples with the added requirements that the date of venepuncture and EDD (scan) must be supplied. Testing is normally done at the 16 week appointment.

Testing is performed at the International Blood Grouping Reference Laboratory (IBGRL) in Bristol and takes about 14 days.

Results can be obtained from the national Blood Service sp-ICE server (first choice) and the contact number for the laboratory is 0117 921 7572.