Normal pregnancy and antenatal care: fetal haemoglobin: 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: fetal haemoglobin

Sample item

Fetal blood takes up oxygen from maternal blood across the placenta even though the fetal PaO2 is only about 30 mmHg. Which property of fetal haemoglobin (HbF) mainly explains this?

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

The answer and the reasoning

C. Poor 2,3-bisphosphoglycerate binding, giving higher oxygen affinity

HbF (alpha2 gamma2) binds 2,3-BPG poorly, so its oxygen dissociation curve lies to the left of that of HbA. Fetal blood therefore loads oxygen at the low PO2 found in the intervillous space. Fetal haemoglobin concentration is in fact higher than maternal, which further aids transfer.

Option by option

  • A. Strong 2,3-bisphosphoglycerate binding, giving higher oxygen affinity

    Strong 2,3-BPG binding lowers oxygen affinity, as in adult HbA, and would hinder uptake.

  • B. A lower haemoglobin concentration than the maternal blood has

    Fetal haemoglobin concentration is higher, not lower, than maternal, and this is not the mechanism.

  • C. Poor 2,3-bisphosphoglycerate binding, giving higher oxygen affinity (correct)

    Correct. Weak 2,3-BPG binding by HbF shifts its curve left, so it saturates at lower PO2.

  • D. A rightward shift of the dissociation curve, favouring oxygen release

    A rightward shift would reduce affinity and favour oxygen release to tissue, not uptake from the mother.

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 C, Poor 2,3-bisphosphoglycerate binding, giving higher oxygen affinity. HbF (alpha2 gamma2) binds 2,3-BPG poorly, so its oxygen dissociation curve lies to the left of that of HbA.

Why is "Strong 2,3-bisphosphoglycerate binding, giving higher oxygen affinity" not the answer?

Strong 2,3-BPG binding lowers oxygen affinity, as in adult HbA, and would hinder uptake.

Why is "A lower haemoglobin concentration than the maternal blood has" not the answer?

Fetal haemoglobin concentration is higher, not lower, than maternal, and this is not the mechanism.

Why is "A rightward shift of the dissociation curve, favouring oxygen release" not the answer?

A rightward shift would reduce affinity and favour oxygen release to tissue, not uptake from the mother.

What concept does this question test?

It tests Normal pregnancy and antenatal care: fetal haemoglobin in OBG. Fetal blood therefore loads oxygen at the low PO2 found in the intervillous space. Fetal haemoglobin concentration is in fact higher than maternal, which further aids transfer.

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: fetal haemoglobin?

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.