Physiological changes of pregnancy

Here are some interesting points I picked up at an ASN review to reinforce Gearoids post discussing changes in reference values during pregnancy and Michael's board review post which covers a lot of the physiology also.

Kidneys increase by 1-2cm in length and up to 30 - 70% in volume, due to an increase in both vascular and interstitial compartments.

The collecting system becomes dilated, with up to 80% of women having evidence of hydronephrosis. The right side predominates, as the gravid uterus is more often dextro-rotated. This can predispose to the development of UTIs due to urinary stasis.

Plasma volume steadily rises by 30-50 % up to around 32 weeks, causing a dilutional anaemia. Total body water can increase by 6-8 litres – most women have some clinically detectable edema.

Cardiac output increases, initially via increased heart rate of 10-20%, followed later by an increase in stroke volume of >20% by week 8. This results in an overall increased cardiac output of 40-50% by week 24. However, systemic vascular resistance falls throughout pregnancy, leading overall to a decrease in mean arterial blood pressure of around 10mmHg by the second trimester.

The net result is increased tissue perfusion – GFR increases by up to 50% by the middle of the second trimester, with corresponding decreases in serum creatinine of similar magnitude. This is maintained until the mid-third trimester, after which it begins to decline towards pre-pregnancy levels.

Despite increases in GFR, glomerular blood pressure appears to remain relatively protected by concomitant reductions in afferent and efferent arteriole resistance.

Proteinuria – there is some debate if ‘normal’ amounts of proteinuria during pregnancy exist, or if any proteinuria should be seen as abnormal. Certainly, new proteinuria, progressively increasing proteinuria or levels over 0.3g/day should ring alarm bells. The presence of hypertension with any proteinuria mandates extremely close follow-up and blood pressure control. Return to normal protein excretion after pregnancy may take up to 5 months.

Glucosuria – mild glucosuria can occur in the absence of hyperglycaemia due to increased filtered load and reduced tubular capacity for reabsorption.

Acid-base – a mild chronic respiratory alkalosis usually exists due to normal hyperventilation in pregnancy, caused by progesterone-mediated stimulation of respiratory centres.

Osmolality – a reduction in Posm of 5 - 10mOsm/Kg occurs. The osmotic threshold for AVP release and thirst are reduced (possibly by hCG), allowing maintenance of this new set-point. Note also, the placenta releases vasopressinase, necessitating an increased rate of AVP production. Overall, serum sodium can therefore fall by 4-5 mmol/L during pregnancy.

Finnian Mc Causland MD