Question

Which of the following drug does not cross the placenta?

• A. Warfarin
• B. Heparin
• C. Phenindione
• D. Bromocriptine

Hint:

Heparin is a low molecular weight drug that does not cross the placenta, making it safe for pregnant women.

Answer 1

The Correct Option is B

Placental transfer mainly depends on a drug's molecular size, charge, and lipid solubility.
Warfarin, phenindione, and bromocriptine are all small, lipid-soluble molecules that cross the placenta easily, which is why warfarin and phenindione carry fetal risk.
Heparin, in contrast, is a large, highly charged mucopolysaccharide that cannot cross the placental barrier, making it the anticoagulant of choice in pregnancy.
So the answer is Heparin.

Answer 2

Instead of just recalling the fact, reason from the physical property that governs whether a drug crosses the placenta: small, uncharged, lipid-soluble molecules pass through easily; large, charged molecules do not.

1. Warfarin: A small lipophilic molecule; readily crosses the placenta and is associated with known fetal harm.
2. Heparin: A large, strongly negatively charged polysaccharide; its size and charge prevent placental crossing, which is why it is considered safe with respect to direct fetal drug exposure.
3. Phenindione: Chemically similar in class to warfarin — small and lipid-soluble — so it also crosses the placenta with comparable risk.
4. Bromocriptine: A small molecule able to cross lipid membranes, including the placental barrier.

Applying the size/charge rule consistently across all four options isolates heparin as the one exception that cannot cross into fetal circulation.

Therefore, the correct answer is Heparin.