Frequency of nucleated red blood cells in maternal blood during the different gestational ages

We wished to determine the time of pregnancy at which optimal numbers of nu cleated red blood cells (NRBC) are present in maternal blood. Because 30% o f the NRBC in maternal blood are fetal, there are implications for prenatal screening and diagnosis. Samples of whole blood were collected from each o f 225 women at various times during pregnancy. The samples were processed b y charge flow separation (CFS), the NRBC enumerated, and the numbers compar ed on a week-to-week basis. To quantify the relationship between week of pr egnancy and actual and log-transformed numbers of NRBC recovered, Pearson p roduct moment and Spearman correlation coefficient were estimated for each of four CFS instruments and for the four instruments combined. When the dat a were analyzed, we found no relationship between stage of pregnancy and nu mbers of NRBC recovered. Even after logarithmic transformation. variability among the women, estimated by standard deviation, was large and relatively stable across the different stages of pregnancy. The number of NRBC recove rable by CFS appears to be constant between 7 and 25 weeks.