Fetal blood hemoglobin concentration and erythrocyte, reticulocyte, and erythroblast counts were determined in umbilical cord samples obtained from 194 pregnancies at 17 to 40 weeks' gestation. The fetuses sampled were undergoing prenatal diagnosis and were subsequently found not to be affected by the condition investigated. The hemoglobin concentration and erythrocyte count increased linearly with gestation from respective means of 11 gm/dl and 2.5 x 10(12)/L at 17 weeks to 15.5 gm/dl and 4.5 x 10(12)/L at 40 weeks. The erythroblast count decreased exponentially from a mean of 83/100 leukocytes at 17 weeks to 4/100 leukocytes at 40 weeks. The reticulocyte count decreased linearly from a mean of 27.5 x 10(9)/L or 10/100 red blood cells at 17 weeks to 17.5 x 10(9)/L or 4/100 red blood cells at 40 weeks.