Objective: We evaluated monoclonal antibodies to 3 cell surface and 3 intracellular antigens for their relative usefulness as markers to identify fetal cells in maternal blood. Study Design: With indirect immunocytochemical labeling techniques, antigen expression was studied in 52 fetal blood samples as a function of gestational age, fetal karyotype, the presence of multiple anomalies detectable on ultrasonography, and anemia. Results: A decline in the expression of these antigens as gestational age advanced was demonstrated. Samples from karyotypically abnormal fetuses, fetuses with multiple anomalies, and anemic fetuses showed an antigenic distribution that was immature for gestational age. In normal fetuses ζ globin and ϵ globin expression decreased after 12 to 14 weeks, potentially limiting the utility of these proteins as fetal cell markers in the isolation of fetal cells from maternal blood. Conclusions: The results of this study demonstrate a fetal developmental hematologic profile that varies with gestational age and also with pathologic condition. Antibodies to the γ chain of fetal hemoglobin and the transferrin receptor (CD71) are the most useful fetal cell–identifying reagents. (Am J Obstet Gynecol 1999;180:1234-9.)