‘In pregnancy everything gets bigger’. Both kidneys increase in size by 1–2 cm. GFR increases by up to 50 % in the second trimester. There is a corresponding fall in serum creatinine which therefore becomes a less useful marker of acute kidney injury i.e. a pregnant patient can lose 50 % of her renal function and still have serum creatinine that is well within the normal range. Hormonal and pressure effects cause dilatation of the ureters and renal pelvis which is often more marked on the right side, from the second trimester onwards. This increases the risk of ascending infection (Fig. 34.1).

The risk of ascending urinary infection is increased for the reasons given above. Acute pyelonephritis occurs in up to 25 % pregnant women with asymptomatic bacteriuria and because of this antibiotic treatment is indicated. More importantly there is a risk that renal function will deteriorate that is related to serum creatinine concentration at the start of pregnancy. Renal function hardly ever deteriorates if baseline creatinine is less than 125 μmol/l but frequently does if baseline creatinine is higher than this (Box 34.1). Patients with proteinuric nephropathy will almost always develop heavier proteinuria during pregnancy. For some this may mean nephrotic range proteinuria leading to significant oedema. Proteinuria tends to return to baseline values after delivery.

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