Alterations in kidney physiology have long been sought to explain certain phenomena of normal pregnancy. Most suggestive of a renal disturbance was the well known tendency of pregnant women to retain salt and water, clinical evidence of which is found in the edema of late pregnancy and in the diuresis of the first few days postpartum. There was also the apparent readiness with which pregnant women developed specific toxemia with its proteinuria. It had indeed been suggested that certain types of toxemia were caused by the alleged strain of pregnancy imposed on the "low reserve" kidney (1).Additional interest has been given this subject by recent observations that the steroid hormones of the testis and the ovary cause an increased retention of salt and water (2, 3, 4, 5) and when injected into rats and mice lead to a considerable increase in the size of the kidney (6,7,8). Two of these substances, estrogen and progesterone, are produced by the placenta and are present in great concentration in the body fluids of all pregnant women. Good reasons therefore exist for considering the possibility of change in kidney physiology, although no consistent abnormality in renal function has yet been demonstrated in normal pregnancy (9, 10, 11).The present report concerns the measurements of glomerular filtration, effective renal blood flow and the total functioning tubular tissue in normal pregnant women. Tests were carried out during pregnancy and were repeated after delivery, so that the effects of delivery on the kidney function of a single individual can be studied.The rate of filtration at the glomerulus is determined by the inulin clearance, the rate of effective renal blood flow by the diodrast clearance, and the 1This study was made with the aid of a grant from the Commonwealth Fund. maximum tubular excretory function or tubular excretory mass by diodrast Tm. The physiological basis for these measurements has been reviewed by Smith and his associates (12,13,14,15). Smith has also recently discussed the functional significance and limitations of renal clearances (16).
MATERIALFor the tests patients were selected from the prenatal clinic or the ward. Fifteen women were studied in the last lunar month, three in the ninth month, and three in early pregnancy. Nine of these twenty-one were again tested after delivery. None had hypertension, proteinuria, edema, or a history of kidney disease, hypertension or specific toxemia of pregnancy.
METHODThe patient was given a liter of water to drink the evening before the test. The next morning between 7:30 and 8:30 she was given another liter. No breakfast was allowed.Inulin, phenol red and diodrast were given by continuous intravenous infusion at a controlled rate. When this work was begun in May, 1938, physiological saline was used in the infusion fluid. At the same time, tests were started on patients with specific toxemia of pregnancy in whom it was soon found that an adequate urine flow could not be obtained by this method. A two per cent solution of sodium sulphate (anhydrou...