Single Best Answer (SBA) · Maternal Physiology in Pregnancy

A clinician explains by how much plasma volume increases in normal pregnancy. Which single figure is correct by term?

Choose one answer.

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Answer: A. About 30-50% above the pre-pregnancy volume

Plasma volume expansion is a hallmark cardiovascular change.

Plasma volume increases by about 10-15% by 6 weeks, rising to 30-50% above the pre-pregnancy volume by term.
The red cell mass also rises but less than the plasma, producing the dilutional (physiological) anaemia.

System	Key changes in pregnancy
Cardiovascular	Plasma volume +30-50%; cardiac output +40% (mainly stroke volume, also heart rate); systemic vascular resistance falls; mid-trimester BP dip
Respiratory	Minute ventilation +30-50% (via tidal volume, not rate); compensated respiratory alkalosis; reduced FRC
Renal	GFR +~50% by 2nd trimester; renal blood flow +80%; lower creatinine/urea/uric acid; glycosuria; increased proteinuria
Haematological	Dilutional anaemia; hypercoagulable (raised fibrinogen and clotting factors)
Gastrointestinal	Reduced LES tone and delayed gastric emptying (reflux); constipation; ptyalism; gingivitis

The expanded volume supports the increased cardiac output and uteroplacental perfusion.
Inadequate plasma volume expansion is associated with pre-eclampsia and growth restriction.

So a 30-50% increase by term is the expected adaptation.

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