Renal physiology of pregnancy

Abstract

Pregnancy involves remarkable orchestration of physiologic changes. The kidneys are central players in the evolving hormonal milieu of pregnancy, responding and contributing to the changes in the environment for the pregnant woman and fetus. The functional impact of pregnancy on kidney physiology is widespread, involving practically all aspects of kidney function. The glomerular filtration rate increases 50% with subsequent decrease in serum creatinine, urea, and uric acid values. The threshold for thirst and antidiuretic hormone secretion are depressed, resulting in lower osmolality and serum sodium levels. Blood pressure drops approximately 10 mmHg by the second trimester despite a gain in intravascular volume of 30% to 50%. The drop in systemic vascular resistance is multifactorial, attributed in part to insensitivity to vasoactive hormones, and leads to activation of the renin-aldosterone-angiostensin system. A rise in serum aldosterone results in a net gain of approximately 1000 mg of sodium. A parallel rise in progesterone protects the pregnant woman from hypokalemia. The kidneys increase in length and volume, and physiologic hydronephrosis occurs in up to 80% of women. This review will provide an understanding of these important changes in kidney physiology during pregnancy, which is fundamental in caring for the pregnant patient.

Figure 1

Hormonal variation throughout pregnancy and early postpartum. Mean data are of plasma human chorionic gonadotropin, estradiol, progesterone, and relaxin levels, before, during, and 6 weeks after pregnancy. Adapted from Ogueh and his colleagues.

Figure 2

Percentage change from baseline of GFR and RPF throughout pregnancy and postpartum. Mean values are measured by inulin or isothalamate and p-aminohippurate clearance for GFR and RPF, respectively, as synthesized by Odutayo and Hladunewich.