BACKGROUND: Cell-free fetal DNA in maternal plasma opens the way for routine risk-free diagnosis of fetal D status of D– mothers. The focus was on accuracy of RHD typing and confirmation of fetal DNA in maternal plasma while RHD was not detected.

STUDY DESIGN AND METHODS: Plasma DNA was extracted (by manual and/or automatic method) from 255 D– pregnant women and amplified in exons 7 and 10 and intron 4 of RHD gene with real-time polymerase chain reaction. The presence of fetal DNA was confirmed by testing SRY and, when negative, by one of 11 different polymorphisms found in the father but not in the mother. The results were compared with the D status of the newborns.

RESULTS: After exclusion of 25 cases (10%) because of material shortage, in 230 cases (90%) available for complete study, the predictive value of the procedure of fetal RHD testing (RHD genotyping plus confirmation of fetal DNA) was 99.6 percent. SRY detection confirmed fetal DNA presence in maternal plasma in all boys, whereas the detection of various polymorphisms in all girls but one.

CONCLUSIONS: Fetal RHD genotyping from maternal plasma may be used with confidence, although additional polymorphisms for confirmation of fetal DNA should be included for 100 percent predictive value (instead of 99.6%).