Objective: To determine if the RhD status of the fetus can be detected in an unselected group of RhD-negative women with ongoing pregnancies booking for routine antenatal care.
Material and methods: We obtained 2.5 ml of whole blood from 202 unselected women with a normal ongoing pregnancy who booked for antenatal care before 20 weeks' gestation. DNA was extracted and real time quantitative PCR performed. Primers and a specific probe were targeted at the Rhesus D gene.
Results: Of 194 women, 31 were RhD-negative with no evidence of prior isoimmunisation. They delivered 17 RhD-positive and 14 RhD-negative babies. Of the 17 RhD-negative women carrying a RhD-positive fetus, the RhD gene was detected in the maternal blood in 14 (82%). There were no false positives. The quantity of RhD gene detected in the maternal blood of RhD-negative mothers ranged from 2 x 10(2) to 4 x 10(6) copies/ml of whole blood.
Conclusion: Using real time quantitative PCR assays to amplify free fetal DNA in the maternal circulation, identification of RhD-positive fetuses in RhD-negative mothers is feasible when they book for antenatal care. Before such RhD genotyping can be introduced into clinical practice, however, further studies are required to show that false negative results can be eliminated and to show that this diagnostic test is reliable outside a research setting.