A tremendous number of data on physiochemical changes in the blood during pregnancy have been published, but, with one or two exceptions, the observations reported are based on groups of patients at different periods and not on the same patient throughout pregnancy and the puerperium. Realizing that the normal must be known before abnormalities can be detected, we began, three years ago, a systematic study of the blood of a number of women early in pregnancy and followed them into the puerperium. We also took a number of patients at term and followed them for eight weeks, or longer, after delivery. The reason for the latter series was the impossibility of getting patients before the tenth week of pregnancy, when there had already been some change in the blood, and, furthermore, the difficulty in persuading patients to submit to repeated studies, both before and after delivery, not to mention the