**Core Concept**
Placental transfer of drugs depends on their lipid solubility, molecular weight, and ability to pass through the chorionic barrier. Among anticoagulants, warfarin is highly lipid-soluble and readily crosses the placenta, exposing the fetus to therapeutic levels.

**Why the Correct Answer is Right**
Warfarin is a highly lipid-soluble vitamin K antagonist that easily crosses the placenta via passive diffusion. It binds to maternal plasma proteins but still reaches the fetal circulation, where it can cause fetal bleeding and coagulopathy. This property makes it the most placenta-crossing anticoagulant among the listed options. Its use during pregnancy is associated with a high risk of fetal warfarin syndrome.

**Why Each Wrong Option is Incorrect**
Option A: Heparin does not cross the placenta due to its high negative charge and poor lipid solubility; it remains confined to maternal circulation and does not affect the fetus.
Option C: Dicumarol is a less potent anticoagulant with limited placental transfer compared to warfarin and is not significantly absorbed into fetal circulation.
Option D: Nicoumalone (a misnomer; likely meant as "acenocoumarol") is less lipid-soluble than warfarin and has minimal placental transfer.

**Clinical Pearl / High-Yield Fact**
Warfarin is the only anticoagulant known to cross the placenta significantly and cause fetal bleeding. Avoid it in pregnancy; use low-molecular-weight heparin (LMWH) instead. Always monitor INR and fetal well-being closely.

✓ Correct Answer: B. Warfarin