How Pregnancy Affects Kidney Function
Normal pregnancies have complex effects on the kidneys and on hemodynamics—changes that bear keeping in mind when evaluating pregnant patients with chronic kidney disease, Dr. August said.
During pregnancy, kidney size, blood flow, and glomerular filtration rate (GFR) all increase. GFR increases by as much as 50%, said Dr. August, of Cornell University in New York.
Renal vasodilation leads to increases in renin, urinary protein, aldosterone excretion, sodium and water reabsorption, glycosuria, and aminoaciduria. There are concomitant decreases in uric acid reabsorption and serum creatinine, sodium, and osmolarity. Normal serum sodium for a pregnant woman is in the 135–137 mmol/L range. Likewise, serum creatinine levels can be as low as 0.5 mg/dL and is usually 0.6 mg/dL; levels above 0.8 mg/dL indicate a decreased GFR, she said.
Uric acid levels also are lower in pregnant women—something to keep in mind when evaluating patients for possible preeclampsia, which causes serum uric acid levels to rise. In the second trimester, the normal uric acid level is usually not much higher than 3.5 mg/dL, and in the third trimester, it can go up as high as 4.5 mg/dL. “Anything higher than that is not quite normal,” Dr. August said.
Dilation of the urinary tract in pregnancy increases susceptibility to urinary tract infections. She recommends monthly screening for asymptomatic bacteriuria in pregnant women who are prone to urinary tract infections. Acute pyelonephritis can be much more serious than usual in pregnant women.
Hemodynamic changes during pregnancy include decreased blood pressure and increased cardiac output, heart rate, and respiratory rate. Some of these changes can actually be helpful for women with chronic kidney disease. Women who have been treated for hypertension and/or chronic kidney disease before pregnancy may require fewer medications during pregnancy.