15th July 2024

Renal physiology

During pregnancy, there is a physiological increase in glomerular filtration rate (GFR), secondary to an increase of approximately 60% in renal blood flow due to increased cardiac output and blood volume. As a result, in pregnant women with normal renal function, creatinine decreases in the first trimester. This can occur as early as the 5th week of pregnancy. During the last few weeks of pregnancy, GFR falls, and therefore creatinine increases, so that renal function returns to baseline shortly postpartum, although GFR can be reduced for up to the first 3 months postpartum. A creatinine above 75µmol/l (0.87mg/dl) should be considered abnormal in pregnancy, indicating renal impairment either secondary to pre-existing renal disease or acute kidney injury. In pregnancy, estimated GFR (eGFR) is not a reliable marker of kidney function.

The increase in GFR in pregnancy can lead to glycosuria in 50% of pregnant women, even with a normal blood sugar, as the kidney tubules are unable to reabsorb all glucose from the filtrate back into the blood. Similarly, mild proteinuria (up to 300 mg over 24 hours) can also occur in pregnancy due to the same mechanism. However, a urine protein creatinine ratio (uPCR) above 30 mg/mmol or a urine albumin creatinine ratio (uACR) above 8 mg/mmol warrants urgent referral to obstetrics.

Other changes during pregnancy occur in the anatomy of the urinary tract itself. The smooth muscle of the renal pelvis relaxes, leading to dilation and a physiological hydronephrosis that requires no intervention or treatment.