**Core Concept**
Heparin is a type of anticoagulant that works by enhancing the activity of antithrombin III, a naturally occurring inhibitor of thrombin. Unlike other anticoagulants, heparin is a large, negatively charged molecule that does not readily cross the placenta due to its size and charge.

**Why the Correct Answer is Right**
Heparin's large molecular size and negative charge prevent it from crossing the placental barrier. In contrast, other anticoagulants like warfarin, dicoumarol, and nicoumalone can cross the placenta and potentially affect fetal development. Warfarin, for example, can cause fetal warfarin syndrome, characterized by nasal hypoplasia, stippled epiphyses, and other skeletal abnormalities.

**Why Each Wrong Option is Incorrect**
**Option B:** Warfarin is a small, lipophilic molecule that can easily cross the placenta and affect fetal development.
**Option C:** Dicoumarol is a coumarin derivative that works similarly to warfarin and can also cross the placenta.
**Option D:** Nicoumalone (acenocoumarol) is another coumarin anticoagulant that can cross the placenta and potentially cause fetal harm.

**Clinical Pearl / High-Yield Fact**
When choosing an anticoagulant for a pregnant patient, heparin is often the preferred option due to its lack of fetal toxicity. However, it requires careful monitoring of the patient's international normalized ratio (INR) to prevent bleeding complications.

**✓ Correct Answer: A. Heparin**