Sodium balance during pregnancy in the rat.

The purpose of this study was to quantitate sodium balance during gestation in the rat (3 wk) to determine the time course of salt retention and to define the distribution of the retained salt. We have found that while sodium retention appears to begin during the 2nd wk, the bulk of retention occurs during the 3rd wk when the rat excretes only 67% of that ingested. Compared to that of nonpregnant, control rats, net retention for all of gestation was 9.8 meq. Despite a significantly higher sodium intake in pregnant rats, neither urinary nor fecal sodium excretion was different in pregnant and control rats. Fifty-nine percent of the retained sodium was found in the products of conception. Inulin space was increased by term from 19.7 to 25.4% of nonconceptus body weight. In pregnant rats a net retention of 6.3 meq of potassium was noted, 55% of which was found in the products of conception. We conclude that the rat retains sodium and potassium during pregnancy, predominantly in the third trimester. Over half of these salts are found within the conceptus products.