Cell-free DNA fetal blood group testing for RhD-negative pregnant women: Implications for midwifery
Cell-free DNA (cfDNA) RhD testing of maternal plasma has been implemented in Avon as a clinical service, so that normal rhesus D-negative pregnant women (about 15% of pregnant women) are only given antenatal anti-D if the fetus is RhD positive. As a result, only RhD-negative women carrying a RhD-positive baby are given antenatal anti-D, meaning that about 40% of healthy RhD-negative pregnant women need not be exposed to a blood product that is pooled from many donors. This also means health services can conserve the use of a costly product (requiring deliberate sensitisation of volunteers) that can be in short supply, for those who need it. The cfDNA RhD test is now available in NHS laboratories and this article proposes that maternity services should implement this approach across the country.
Red cell (D) allo-immunisation is a complication of pregnancy that can occur when a pregnant woman does not have the D antigen on her red blood cells (RhD negative) but her fetus does (RhD positive). When fetal blood cells pass into the maternal blood during pregnancy, an immune response can be triggered that produces anti-D antibodies, a process called sensitisation. Anti-D is an immunoglobulin G (IgG) antibody that passes across the placenta from mother to fetus and may result in haemolytic immune destruction of fetal red blood cells. Possible fetal complications from this process may include anaemia, hydrops fetalis and intrauterine death (haemolytic disease in the fetus and newborn (HDFN)) (Hadley and Soothill, 2005). A fetus of a sensitised primigravid woman may not be severely affected, but that becomes increasingly likely in subsequent pregnancies as the immune response becomes stronger. Therefore, women found to have RhD sensitisation should be referred to a fetal medicine unit without delay. The placenta is able to metabolise and transfer the bilirubin produced by fetal haemolysis to the mother during pregnancy, but after birth, haemolysis in the neonate may lead to the development of hyper-bilirubinaemia, and severe jaundice may result in kernicterus brain damage. After a neonate has been affected, the disease severity usually increases in each subsequent pregnancy with a RhD-positive fetus, related to the anti-D concentration in the mother's blood.
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