Despite total body volume expansion, the kidney senses a relative "under-filling" and the plasma renin-angiotensin-aldosterone system (RAAS) is activated. This provides a checks and balances system to the massive systemic vasodilation of pregnancy.

  In the end, systemic blood pressure falls, despite the increasing cardiac output, with a nadir at about 16 to 20 weeks' gestation.  

Throughout gestation, the glomerular filtration rate (GFR) increases by 50% and renal plasma flow by 50% to 85%.

  The increased GFR is caused by renal vasodilatation, mediated in large part by the ovarian hormone relaxin. GFR rises by 25% by the fourth gestational week, peaking by week 9. Because of this increase, serum blood urea nitrogen (BUN) and creatinine concentrations fall. 

The change in GFR has important implications for patients with either preexisting or de novo proteinuria. As GFR increases, proteinuria increases. Clinicians should expect this as pregnancy progresses and not be alarmed. However, it is concerning if the level of proteinuria starts to change rapidly. 

Three mechanisms exist to prevent albumin/protein loss by filtration in the normal kidney (Figure 1).

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Renal disease and preeclampsia are your two biggest concerns. An expert nephrologist outlines the steps involved in differential diagnosis and management.