What happens to the CO2 threshold in pregnancy?

• A. Increases
• B. Decreases
• C. Unchanged
• D. Doubles

Answer: (B)

In pregnancy, the respiratory drive becomes more sensitive to CO2 because progesterone stimulates the brainstem respiratory centers. This makes ventilation respond at lower CO2 levels, so the threshold for triggering breathing decreases. As a result, arterial CO2 (PaCO2) falls—typically from about 40 mmHg toward the mid-30s—leading to a mild respiratory alkalosis. The kidneys compensate over time by excreting bicarbonate, which helps bring pH back toward normal. So the CO2 threshold for driving respiration decreases in pregnancy.