Utilizing an easy and safe procedure for fetal blood sampling in utero we have studied 409 fetuses for prenatal diagnosis of rubella, toxoplasmosis, hemophilia, and hemoglobinopathies. Retrospectively we selected 163 fetuses confirmed as normal at birth and tested between 18 and 30 wk of gestation to establish normal hematological parameters and to follow the evolution of erythropoiesis, differential counts, hemoglobin synthesis, and hemostasis. Total white blood cell and platelet counts did not change during this period. The lymphocytes represented the main population and we observed a decrease of normoblasts during gestation. The results show a progressive increase of red blood cells and hemoglobin. This evolution is demonstrated by the ratio hemoglobin A to acetylated hemoglobin F. No significant modification of hemostasis was observed over a 12-wk intrauterine gestation. These results provide useful reference values for future investigations.