Maternal hyperventilation during pregnancy results in a state of mild respiratory alkalosis. What is the correct arterial blood gas pattern expected in the third trimester?

• A pH 7.44, PaCO2 40 mmHg, HCO3− 24 mEq/L (normal)
• B pH 7.35, PaCO2 50 mmHg, HCO3− 28 mEq/L (respiratory acidosis)
• C pH 7.44, PaCO2 30 mmHg, HCO3− 20 mEq/L (compensated respiratory alkalosis) ✓
• D pH 7.50, PaCO2 28 mmHg, HCO3− 24 mEq/L (uncompensated respiratory alkalosis)

Explanation

Progesterone acts as a respiratory stimulant, increasing tidal volume and driving minute ventilation up ~40–50% in pregnancy. This lowers PaCO2 to ~30–32 mmHg (respiratory alkalosis). The kidney compensates by excreting HCO3−, reducing plasma HCO3− to ~18–22 mEq/L, and pH returns toward but slightly above 7.40 (~7.42–7.44). This mild compensated respiratory alkalosis is normal in pregnancy and facilitates CO2 transfer from fetus to mother across the placenta (by maintaining a lower maternal PaCO2 as the 'sink').

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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