Fetal cells exist in maternal blood and can be utilized for prenatal genetic diagnosis. These cells are present during the first and second trimesters, with frequency increasing as gestation advances. Enrichment of erythroblasts by various density gradient techniques and either magnetic activated or flow sorting techniques can be followed by FISH with chromosome-specific DNA probes. This approach has allowed detection of trisomy 21, trisomy 18, Klinefelter syndrome 47,XXY and 47,XYY. Polymerase chain reaction (PCR) analysis of maternal blood has enabled the detection of fetal sex, certain Mendelian disorders (e.g. β-globin mutations), HLA polymorphisms, and fetal Rhesus (D) blood type. The fetal cell types receiving the most attention has been nucleated erythrocyte (erythroblast) and the trophoblast. Lymphocytes and granulocytes are also present in maternal blood; however, lymphocytes are considered the cell type most likely to persist after pregnancy A large scale collaborative is now underway in the U.S. that will allow us to determine whether sensitivity and specificity of this technique provide a noninvasive alternative to conventional methods of prenatal cytogenetic diagnosis.