Innumerable articles about the anemia of pregnancy have been published, but, although these reports presuppose a knowledge of the normal changes in the blood incident to pregnancy, so far as we know, complete hematologic studies made on the same women throughout pregnancy are not available. Therefore, as a preliminary to an investigation of the true anemia of pregnancy, we made serial hematologic studies of the blood of the same women during pregnancy and the puerperium. Haden,1 Osgood and Haskins,2 Wintrobe3 and others have determined the variations and averages in normal nonpregnant women. Their work demonstrates that the average erythrocyte has a definite volume and a definite content of hemoglobin, and that the hemoglobin forms a definite proportion of the cell. They discussed the importance of the color index, but stated that in addition the volume and saturation indexes together with the data for the average cell are