Ultrasound guided fetal blood sampling performed for diagnostic purpose has allowed us to determine the evolution of normal hematological parameters i.e. all nucleated cells, red blood cells, platelets, hemoglobin, mean corpuscular volume in 2,680 normal fetuses from the 18th to the 30th week of gestation. Differential blood counts were estimated. We determined normal circulating level of erythropoietic progenitors cells, which is three times higher than in cord blood at birth. Erythropoietin levels were very low and were not modified during the second and the third trimester of pregnancy. All these new data reveal fundamental quantitative differences in the hematopoietic system of fetal on adult subjects.