[New approaches to prenatal diagnosis of rhesus incompatibility]

Despite generalisation of anti-D immunoprophylaxis, RhD allo-immunisation still remains the major cause of severe haemolytic disease of the fetus and of the newborn (HDFN). The routine follow up of pregnant women comprises: the ABO/D, Rh/Kell red cells typing and the search for irregular antibodies. In case of anti-D immunisation, the paternal Rh phenotype, when known, provides useful information regarding the probability for the fetus to have inherited the D antigen and thereby to be exposed to the risk of HDFN. The antibody titre, which is predictive of possible in vivo haemolysis, must be interpreted in the light of the previous obstetric history, and can lead to the decision of invasive amniocentesis. Then the measurement of the optical density (deltaOD450 nm) and the fetal RhD typing can be realised on amniotic fluid. New molecular techniques make it possible now to demonstrate the presence of fetal DNA in maternal plasma. These methods lying on non invasive procedures could advantageously be applied to the genotyping of fetal RHD during pregnancy. The present paper aims to discuss the predictive values of RHD fetal genotype in maternal plasma of RhD negative mothers. The ante-partum management of immunised pregnant women is reviewed in the light of this new molecular approach combined to Doppler ultrasonography of the fetal middle cerebral artery. This non invasive method for determining fetal RHD genotype could be systematically proposed to all RhD negative pregnant women for a better targeted prenatal follow-up and an increased efficacy of RhD prophylaxis.