- Objective: To investigate whether the transfer of fetal blood cells to the maternal circulation is perturbed in pregnancies affected by preeclampsia. Methods: Fetal erythroblasts were isolated from eight women with clinically diagnosed preeclampsia (blood pressure values of at least 140/90 mmHg and associated proteinuria) and an equal number of matched corresponding controls. All patients in both groups were pregnant with male singleton fetuses. The presence of fetal cells was evaluated histologically and by fluorescence in situ hybridization for X and Y chromosomes. Results: The number of fetal cells was higher in preeclamptic patients than in controls, with respect to both nucleated red blood cells (median per 200 cells 38 versus 7; P < .001) and the proportion of these cells that were of fetal origin (median per 2000 cells 9 versus 2; P = .001). Conclusion: These results suggest that the trafficking of fetal cells into the maternal periphery is disturbed in patients with preeclampsia. Because it is unlikely that such an altered flow of cells is restricted to the erythroblasts examined in this study, these findings also may lead to interesting new concepts regarding the development of preeclampsia and possibly the associated syndrome of hemolysis, elevated liver enzymes, and low platelets.