In recent years, considerable progress has been achieved in the genetic analysis of fetal cells isolated from maternal blood. The goal of clinical diagnosis of either fetal chromosome abnormality or Mendelian DNA mutation has already been realized in an ever-increasing number of cases. The remaining challenges involve increasing the yield and visual identification of fetal cells while decreasing the remaining numbers of maternal cells. It is presently unknown whether the frequency of fetal cells in maternal blood differs in cytogenetically abnormal pregnancies as compared with normal pregnancies. Although fetal cells in maternal blood have a practical role in prenatal diagnosis, their very existence and persistence in the mother raise many new questions regarding maternal-fetal interaction.