Renal System

The renal system undergoes major changes in pregnant patients, mainly because of the effects of progesterone and the mechanical effects of compression of the enlarging uterus. Urea, creatinine, and uric acid clearance all rise in pregnancy (as illustrated in Table 58-4 ). Renal plasma flow and the glomerular filtration rate (GFR) both increase rapidly in pregnancy as a result of the increase in cardiac output. The GFR rises by almost 50%; this increase, accompanied by the dilutional effect of plasma volume expansion, accounts for the decrease in plasma creatinine and urea. Hence, "normal" renal indices in pregnancy are lower than in the nonpregnant state. Therefore, blood urea nitrogen and creatinine levels that would be considered marginally elevated in prepregnant patients are usually indicative of severe renal impairment in parturients. The increase in GFR generally precedes the expansion of blood volume and is considered to be a marker of pregnancy-induced vasodilation.^[27] Glycosuria is a common finding that is attributable to the increase in GFR and reduced renal tubular resorption capacity.