In order to evaluate fetal erythropoiesis we measured red blood cells, hemoglobin, hematocrit, serum transferrin receptor (sTfR), and iron status parameters in fetuses undergoing percutaneous umbilical blood sampling, and in normal newborns at term. We found high levels of sTfR in fetuses and newborns as compared with normal adults (3,149 +/- 181 vs. 1,881 +/- 137 ng/ml, P < 0.00001). Concentrations of sTfR correlate with gestational age and red blood cell numbers (r = 0.441, P < 0.001; r = 0.366, P = 0.06). sTfR concentrations do not show correlation with iron status parameters. The increased sTfR concentration is consistent with the fact that fetal life is characterized by cell proliferation and tissue growth. sTfR concentration correlates with gestational age and numbers of red blood cells, and can therefore be considered a good indicator of fetal erythropoiesis. It is conceivable that, during intrauterine life, sTfR expression is independent from iron status. sTfR determination will help in reaching a better understanding of some aspects of fetal physiology, and will help elucidate the physiopathology of fetal hematological diseases.