Our cumulative experience continues to validate the fetal nucleated erythrocyte as the target fetal cell type of choice, primarily because it reflects the cytogenetic status of the current pregnancy. Additional cell types, such as the granulocyte, await further study. Quantitative PCR is a sensitive and useful new method that can facilitate rapid comparisons between cell separation methods or different monoclonal antibodies. It can also be used on patient material to determine final purity of the enriched maternal samples. If the purity is too low, FISH studies will be complicated by the presence of thousands of maternal cells. Our planned studies include an analysis of why aneuploid pregnancies appear to have a higher number of fetal cells in the maternal circulation. We are also studying the timing of the fetomaternal transfer of cells with qPCR analysis of sorted maternal samples drawn weekly from well-dated women. We are continuously improving our methods (both in separations and antibodies) to reach a fetal cell purity of at least 20% for cytogenetic diagnosis by FISH studies. With the knowledge obtained thus far by us and by others, such a goal appears to be achievable within the near future.