Expansion
GFR rises by about 50 per cent, so creatinine and urea fall
Expansion
Increases
- Renal plasma flow by 60 to 80 per cent
- GFR by about 50 per cent, from early in the first trimester
- Kidney size, by about 1 cm
- Physiological hydronephrosis and hydroureter, right more than left, from progesterone and mechanical compression
Decreases
- Creatinine to about 50 to 70 micromol/l, and urea falls similarly
- Serum sodium falls by about 5 mmol/l, with a resetting of the osmostat
- Serum bicarbonate falls, compensating the respiratory alkalosis
Other consequences
- Glycosuria is common, since the increased filtered load exceeds the tubular maximum
- Mild proteinuria up to 300 mg a day is normal
- Increased risk of urinary tract infection and pyelonephritis, from stasis, which is why asymptomatic bacteriuria is screened for and treated
The practical point is that reference ranges shift. A creatinine of 90 micromol/l would be normal outside pregnancy but indicates significant impairment within it, and new proteinuria after 20 weeks with hypertension suggests pre-eclampsia.