During normal pregnancy, maternal minute ventilation increases by approximately 40-50% as early as the first trimester. The primary driver of this change is:

• A Rising estrogen levels stimulating the respiratory center
• B Mechanical elevation of the diaphragm
• C Increased oxygen consumption by the fetus
• D Progesterone-mediated increase in respiratory center sensitivity to CO2 ✓

Explanation

Progesterone acts on the medullary respiratory center to increase its sensitivity to CO2, leading to hyperventilation. This occurs as early as the first trimester, before significant mechanical changes from the gravid uterus. The resulting respiratory alkalosis, with PaCO2 falling to approximately 30 mmHg, facilitates CO2 transfer from fetus to mother at the placenta. Estrogen contributes to other gestational changes but is not the primary driver of the increased minute ventilation.

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

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Written and medically reviewed by the StethoPrep medical team.