Renal and Respiratory Changes in Pregnancy
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Direct answer
Renal and respiratory changes in pregnancy include a roughly 50 per cent rise in glomerular filtration rate and a fall in PaCO2 to about 28 to 32 mmHg from hyperventilation. These changes lower serum creatinine and urea, and they also cause physiological glycosuria and hydronephrosis. Recognising them prevents a normal pregnancy from being misdiagnosed as renal disease or respiratory failure.
What you must remember
- Glomerular filtration rate: rises by about 50 per cent early in pregnancy, which lowers serum creatinine and urea.
- Creatinine threshold: a value above about 0.8 mg/dL should prompt a search for renal disease, since the non-pregnant range misleads.
- Glycosuria: common because the filtered glucose load exceeds tubular reabsorptive capacity, and mild glycosuria alone does not diagnose diabetes.
- Proteinuria: physiological proteinuria stays below about 300 mg in 24 hours, and more suggests pre-eclampsia or renal disease.
- Hydronephrosis: physiological, mainly on the right, from progesterone-mediated smooth muscle relaxation and uterine compression of the ureters.
- Ventilation: tidal volume rises and minute ventilation increases by roughly 40 per cent, which is the basis of the hyperventilation.
- PaCO2: falls to about 28 to 32 mmHg, a compensated respiratory alkalosis with pH kept near normal by renal bicarbonate excretion.
- Lung volumes: functional residual capacity falls by about 20 per cent, while inspiratory capacity rises and vital capacity is essentially unchanged.
Why the kidneys and lungs behave differently in pregnancy
Both systems change early and stay altered through pregnancy, so most findings are predictable. Exam questions reward the mechanism, because it explains why each value shifts.
- Progesterone effect: progesterone relaxes ureteric smooth muscle and stimulates the respiratory centre, which explains both hydronephrosis and hyperventilation.
- Renal filtration: renal plasma flow and GFR rise early, so more creatinine and urea are cleared and their serum levels fall.
- Glucose handling: the filtered glucose load exceeds the tubular maximum in many women, so glucose appears in urine even when blood sugar is normal.
- Ureteric dilatation: the ureters widen, usually on the right because of dextrorotation of the uterus, and this causes hydronephrosis that rarely needs intervention.
- Ventilation drive: tidal volume and minute ventilation rise, so the woman breathes more deeply and lowers her PaCO2.
- Lung volumes: the diaphragm rises as the uterus enlarges, reducing functional residual capacity and expiratory reserve volume, while inspiratory capacity increases.
- Acid-base compensation: the kidneys excrete bicarbonate to compensate for respiratory alkalosis, so pH stays near normal.
Where the exam tries to catch you
Examiners set traps by quoting the non-pregnant reference range, so compare each value with the pregnancy norm.
- Trap: accepting a creatinine of 0.9 mg/dL in pregnancy as normal Correct: pregnancy lowers creatinine, so that value is high relative to the pregnancy norm and needs assessment of renal function.
- Trap: diagnosing diabetes from glycosuria Correct: glycosuria alone is physiological, so check blood glucose, fasting or after a glucose load, before labelling diabetes.
- Trap: operating for hydronephrosis without clinical features Correct: physiological hydronephrosis is common, and intervention is reserved for infection, pain or obstruction with renal impairment.
- Trap: reading a PaCO2 of 30 mmHg as respiratory failure Correct: this is the normal pregnancy value with a normal pH, so it is physiological.
Frequently asked questions
Why does serum creatinine fall in normal pregnancy?
Serum creatinine falls in normal pregnancy because glomerular filtration rate rises by about 50 per cent, so more creatinine is cleared from the blood. A creatinine level that looks normal in a non-pregnant woman may therefore signal reduced renal function during pregnancy.
What causes physiological hydronephrosis in pregnancy?
Physiological hydronephrosis in pregnancy is caused by progesterone-mediated relaxation of ureteric smooth muscle and mechanical compression of the ureters by the enlarged uterus. It is usually right-sided because the uterus is dextrorotated, and it generally resolves after delivery.
Why does glycosuria occur in pregnancy without diabetes?
Glycosuria occurs in pregnancy without diabetes because the glomerular filtration rate rises, so the filtered glucose load exceeds the tubular reabsorptive capacity. Glycosuria alone does not diagnose diabetes, and blood glucose testing is needed before any diagnosis is made.
What is the normal PaCO2 in pregnancy?
The normal PaCO2 in pregnancy is about 28 to 32 mmHg, lower than in the non-pregnant state because of hyperventilation. The pH stays near normal because the kidneys excrete bicarbonate, so this is compensated respiratory alkalosis and not respiratory failure.
Which lung volume falls in pregnancy and why?
Functional residual capacity falls by about 20 per cent in pregnancy because the enlarged uterus raises the diaphragm. Expiratory reserve volume falls with it, while inspiratory capacity rises and vital capacity is essentially unchanged.