Normal pregnancy and antenatal care: acid-base in pregnancy: practice question

A OBG question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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OBG · Normal pregnancy and antenatal care: acid-base in pregnancy

Sample item

A healthy woman at 30 weeks, breathing room air, has arterial blood gas values of pH 7.44, PaCO2 30 mmHg, and bicarbonate 20 mEq/L. How should this result be interpreted?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

D. Normal pregnancy: compensated respiratory alkalosis

Progesterone-driven hyperventilation lowers PaCO2 to about 30 mmHg, and the kidneys excrete bicarbonate so that pH stays near normal or slightly alkaline. A PaCO2 of 30 with low bicarbonate and normal pH is therefore physiological, and a PaCO2 of 40 would be relatively abnormal in late pregnancy.

Option by option

  • A. Metabolic acidosis with respiratory compensation

    Metabolic acidosis would have a low pH, not a pH of 7.44.

  • B. Uncompensated respiratory acidosis with hypoventilation

    Respiratory acidosis would show a raised PaCO2 and a low pH.

  • C. Acute pulmonary embolism with hypoxic hyperventilation

    Pulmonary embolism would cause hypoxaemia and symptoms; this healthy woman has a normal pH and no symptoms.

  • D. Normal pregnancy: compensated respiratory alkalosis (correct)

    Correct. Chronic hyperventilation with renal bicarbonate wasting gives a compensated respiratory alkalosis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this OBG question

What is the answer to this OBG question?

The best answer is D, Normal pregnancy: compensated respiratory alkalosis. Progesterone-driven hyperventilation lowers PaCO2 to about 30 mmHg, and the kidneys excrete bicarbonate so that pH stays near normal or slightly alkaline.

Why is "Metabolic acidosis with respiratory compensation" not the answer?

Metabolic acidosis would have a low pH, not a pH of 7.44.

Why is "Uncompensated respiratory acidosis with hypoventilation" not the answer?

Respiratory acidosis would show a raised PaCO2 and a low pH.

Why is "Acute pulmonary embolism with hypoxic hyperventilation" not the answer?

Pulmonary embolism would cause hypoxaemia and symptoms; this healthy woman has a normal pH and no symptoms.

What concept does this question test?

It tests Normal pregnancy and antenatal care: acid-base in pregnancy in OBG. A PaCO2 of 30 with low bicarbonate and normal pH is therefore physiological, and a PaCO2 of 40 would be relatively abnormal in late pregnancy.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Normal pregnancy and antenatal care: acid-base in pregnancy?

Learn management algorithms as steps: PPH, eclampsia, ectopic pregnancy. Then use the flashcards and the other OBG questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.