A clinician explains why serum creatinine and urea are lower than non-pregnant values in normal pregnancy. Which single change is responsible?

Answer: A. Increased glomerular filtration rate

Renal changes in pregnancy

The kidneys undergo marked haemodynamic adaptation.

Key changes

• GFR rises by up to about 50% by the second trimester.
• Renal blood flow increases by up to 80% (from increased cardiac output and renal vasodilatation).

Consequence for blood tests

• The increased filtration lowers serum creatinine, urea and uric acid.
• So 'normal' non-pregnant values may actually be abnormally high in pregnancy.

Other renal effects

• Reduced tubular reabsorption of glucose causes glycosuria; increased urinary protein excretion occurs.
• Sodium balance is maintained by increased tubular reabsorption.

So the lower creatinine and urea reflect a healthy rise in GFR.