A protocol to detect fetal nucleated red blood cells (NRBCs) was tested in 217 pregnant women and in 19 nonpregnant controls. All the pregnant women were sampled after chorionic villus sampling (CVS); 20 were also sampled pre-CVS. NRBC recognition was based upon morphology by using staining of hemoglobin with 3,3-diaminobenzidin (DAB) or by immunocytochemical staining for fetal hemoglobin (HbF). This was combined with FISH analysis for both the X- and Y-chromosomes on the same cells. Progressive refinement of the methods increased the number of cases where NRBCs were detected from 53% (DAB) to 75% and 78% for DAB and HbF staining, respectively, with on average 43 NRBCs (range, 1-220). DAB gave a slightly higher yield than HbF in the lower cell count range (<25). In 6 out of 18 controls, NRBCs were detected with DAB, vs. 1 out of 19 (5%) with HbF. FISH analysis in 41 cases resulted in correct sex prediction in 80% (DAB) and 89% (HbF), respectively. Our data demonstrated an increase of cases with NRBCs (30% to 75%), as well as a rise of the mean number of NRBCs (6 to 29 cells), after CVS. We conclude that DAB staining is a straightforward way to screen for the presence of NRBCs in maternal blood, but is not specific for NRBCs of fetal origin. HbF immunophenotyping is a reliable marker for fetal NRBCs, which detected slightly fewer NRBCs than DAB-staining, but improved sex prediction and significantly reduced false-positive results. CVS at 10-13 weeks of gestation causes a significant increase of NRBCs in maternal blood. These data indicate that further refinement of NRBC detection is needed before application of noninvasive prenatal diagnosis using maternal blood is feasible.