Question

Which metabolic derangement is normally seen in pregnancy?

• A. Metabolic acidosis
• B. Metabolic alkalosis
• C. Respiratory acidosis
• D. Respiratory alkalosis

Hint:

Progesterone drives hyperventilation, so CO2 falls.

Answer 1

The Correct Option is D

Step 1: Start from the hormone change of pregnancy. Rising progesterone resets the brainstem respiratory centre to a more sensitive level, so the pregnant woman breathes more (increased tidal volume).

Step 2: More breathing means more CO2 is exhaled, so arterial pCO2 drops. Since CO2 is an acid gas, removing it shifts pH upward. A primary fall in pCO2 with a rise in pH is by definition a respiratory alkalosis, pointing to option d.

Step 3: The kidney responds to this long-standing alkalosis by dumping bicarbonate in the urine, which is why serum bicarbonate is mildly low and pH stays close to normal (compensation). This rules out metabolic acidosis or alkalosis (the primary problem is respiratory, not renal/metabolic) and respiratory acidosis (CO2 is low, not high).

Step 4: The expected derangement is therefore respiratory alkalosis.
\[\boxed{\text{Respiratory alkalosis}}\]