Which of the following does not cross placenta?

Appeared in: AIIMS Bhatinda NO - 2019

Explanation

Heparin is a large molecule with a high negative charge.
These properties prevent it from crossing the placental barrier and entering the fetal circulation.
Due to its safety profile for the fetus, heparin is the anticoagulant of choice during pregnancy.

Option B: Morphine is a small, lipid-soluble molecule that readily crosses the placenta.
Option C: Naloxone is an opioid antagonist with a low molecular weight that allows it to cross the placenta.
Option D: Warfarin is a small molecule that easily crosses the placenta and is a known teratogen (can cause birth defects).

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Placental Transfer of Drugs as background academic context rather than a clinical decision trigger.
Nurses must always verify the safety of medications before administering them to pregnant patients, considering the potential for placental transfer and fetal effects.
For a pregnant patient requiring anticoagulation (e.g., for DVT or a mechanical heart valve), heparin (or LMWH) is the standard of care because it does not harm the fetus, unlike warfarin.
What if the patient had a mechanical heart valve and was on long-term warfarin therapy before becoming pregnant? The nurse must anticipate an immediate medication change. The patient would be switched from warfarin to heparin for the duration of the pregnancy to prevent fetal harm.

How to Approach the Question

First, identify the core question: It asks which substance does not pass from mother to fetus through the placenta.
Recognize this is a negative-framing question. You are looking for the exception among the four options.
Recall the principles of placental drug transfer: small, lipid-soluble, non-ionized drugs cross easily, while large, charged, protein-bound drugs do not.
Evaluate each option based on these principles: Morphine, Naloxone, and Warfarin are all relatively small molecules known to cross the placenta. Heparin is famously a large, charged molecule.
Conclude that Heparin is the only drug listed that does not cross the placenta.

Concept Tested & Keywords

Concept Tested: Placental Transfer of Drugs
Stem keywords: placenta, cross
Lead-in keywords: does not
Negative lead-in flag: This question uses negative framing ('does not'), requiring you to identify the one option that is an exception.

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E6 Pharmacology Katzung 16e p. 958-960

E6 Midwifery Obstetrics and Gynecology Nursing Sandeep Kaur p. 46-54

Practise the full AIIMS Bhatinda NO - 2019

Attempt every question from this paper in a timed mock, then review the full solution for each one.