Serum Colloid Osmotic Pressure in Normal Pregnancy

Abstract

A slight amount of edema occurs in many women during normal pregnancy. A marked edema or even an anasarca may occur in pre-eclampsia and eclampsia. No adequate explanation has been given for the cause of the edema in normal or toxemic pregnancy.

In the edema of certain types of nephritis, malnutrition, and in some cases of cardiac disease, the cause is found in an abnormally low serum protein concentration. Various studies indicate that edema is likely to occur if the concentration of the serum protein is less than 5.5 gm. % or if the albumin concentration is less than 2.5 gm. %. Dieckmann and Wegner, 1 as well as other investigators, have demonstrated that the concentration of the serum protein in normal pregnancy is at the lower limits of normal. Their average figures obtained from the same women during pregnancy and the puerperium are given in Table I.

The studies of Schade, 2 Govaerts, 3 and Verney 4 would seem to indicate that the oncotic or colloid osmotic pressure of 1 gm. of albumin is 7.54 cm. of water and 1 gm. of globulin is 1.95 cm. cf water. Wells and co-workers, 5 after an extensive study of their methods and comparisons with one devised by himself, conclude that their figures are too high. He stated that “the specific osmotic pressure of serum appears to be a linear function of the albumin concentration. Variations in the globulin concentration over a wide range produce no effect on the specific pressure at constant values of albumin.” The formula for the colloid osmotic pressure derived by him is: P = C (21.4 + 5.9A), where P is the osmotic pressure in millimeters of water, C is the total protein concentration and A is the albumin concentration, in gm. per 100 cc. He found a standard of error of ± 5%.

Runge and Kessler 6 stated that the oncotic pressure was normal in the first trimester of pregnancy. At about the fourth month it began to decrease and steadily decreased until term. Fluctuations occurred during labor and the early puerperium, but it was normal on the eighth postpartum day. Similar changes in the concentration of the serum protein occur and are probably the cause of the alterations in the oncotic pressure.