BACKGROUND : Invasive procedures to obtain fetal DNA for prenatal blood grouping present a risk to the fetus. During pregnancy, cell-free fetal DNA is present in maternal blood. The detection of RHD sequences in maternal plasma has been used to predict fetal D status, based on the assumption that RHD is absent in D– genomes.
STUDY DESIGN AND METHODS : Real-time PCR assays were designed to distinguish RHD from RHD Ψ (possessed by the majority of D– black Africans). Plasma-derived DNA from 137 D– women was subjected to real-time PCR to detect fetal RHD and Y chromosome-associated SRY sequences. The accuracy of RHD genotyping from maternal plasma was investigated by comparing results with those obtained by conventional RHD genotyping from fetal tissue or serologic tests on the infant's RBCs. The quantity of fetal DNA in maternal plasma was investigated in 94 pregnancies.
RESULTS : Fetal D status was predicted with 100- percent accuracy from maternal plasma. The number of copies of fetal DNA in maternal plasma was found to increase with gestation.
CONCLUSION : Combination of the sensitivity of real-time PCR with an improved RHD typing assay to distinguish RHD from RHD Ψ enables highly accurate prediction of fetal D status from maternal plasma. This has resulted in the implementation of a clinical noninvasive fetal RHD genotyping service.