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We have determined the number of fetal nucleated cells and the concentration of cell-free fetal DNA in parallel in the same maternal blood samples either before or after delivery, and studied the relationship between these two.Venous blood samples were taken at four points around delivery from ten women who had singleton male fetus with informed consent. The number of fetal nucleated cells having a Y chromosome specific signal treated by two-color fluorescence in situ hybridization technique was counted using maternal whole blood. The concentration of sex-determining region Y gene sequence was determined using real-time quantitative PCR.The number of fetal nucleated cells decreased after delivery, and some fetal cells were present 1 month after delivery. While cell-free fetal DNA decreased rapidly after delivery and became undetectable 1 day after delivery in eight out of ten cases. The number of fetal nucleated cells did not correlate with the concentration of cell-free fetal DNA in maternal circulation.The present study demonstrates that cell-free fetal DNA disappears very rapidly after delivery and fetal nucleated cells remain longer in maternal circulation, and that there is no correlation between these two either before or after delivery.