Question

Which of the following is a respiratory effect of progesterone during pregnancy?

• A. Increased respiratory drive (hyperventilation)
• B. Decreased tidal volume
• C. Decreased arterial PO₂
• D. Increased arterial PCO₂

Hint:

Progesterone makes the respiratory centre more CO₂-sensitive — the woman hyperventilates.

Answer 1

The Correct Option is A

Step 1: Recall progesterone's action on the respiratory centre. In pregnancy, rising progesterone acts directly on the central respiratory centre and increases its sensitivity to carbon dioxide, stimulating ventilation. This produces a physiological hyperventilation of pregnancy.

Step 2: Trace the downstream changes. The hyperventilation is driven mainly by an increase in tidal volume (roughly +30-40%), which raises minute ventilation. Increased alveolar ventilation blows off CO₂, so arterial PCO₂ falls to about 28-32 mmHg, producing a compensated respiratory alkalosis, and arterial PO₂ tends to rise slightly.

Step 3: Choose the correct option. Increased respiratory drive (hyperventilation) is the direct, defining respiratory effect of progesterone — option 1.

Step 4: Why the others are wrong. Tidal volume increases, not decreases (option 2 wrong). Arterial PO₂ stays normal or rises slightly (option 3 wrong). Arterial PCO₂ decreases because of the increased ventilation (option 4 wrong).

Key fact: Progesterone increases respiratory drive in pregnancy, raising tidal volume and lowering arterial PCO₂ (compensated respiratory alkalosis).