In pregnancy, which arterial blood gas change is typically observed?

• A. PCO2 decreases to 28-32 mmHg
• B. PCO2 increases to 50-60 mmHg
• C. HCO3 increases to 28-30 mEq/L
• D. pH decreases to 7.30

Answer: (A)

In pregnancy, the body shifts toward a lower arterial carbon dioxide level because progesterone stimulates the respiratory drive, increasing ventilation. This chronic respiratory alkalosis typically lowers PCO2 to about 28–32 mmHg. The kidneys then compensate by excreting bicarbonate, so HCO3 decreases rather than increases, and the pH remains near normal or slightly alkaline rather than acidic. So the most consistent ABG change is a decreased PCO2 around 28–32 mmHg. The other options describe CO2 increasing, bicarbonate rising, or acidemia, none of which fit the typical pregnancy pattern.