**Core Concept**
Unfractionated heparin (UFH) is the preferred anticoagulant in pregnancy due to its mechanism of action and safety profile. UFH does not cross the placenta, making it an ideal choice for preventing thromboembolic events in pregnant women.

**Why the Correct Answer is Right**
UFH works by enhancing the activity of antithrombin III, which inhibits thrombin and factor Xa, preventing the formation of blood clots. This anticoagulant effect is reversible, allowing for easier reversal of anticoagulation in case of bleeding complications. Additionally, UFH does not have a significant effect on placental blood flow, making it safe for both the mother and the fetus.

**Why Each Wrong Option is Incorrect**
**Option A:** Warfarin is a vitamin K antagonist that can cause fetal warfarin syndrome, making it contraindicated in pregnancy.
**Option B:** Low molecular weight heparin (LMWH) is a safer alternative to UFH, but it is not the first-line choice due to its higher cost and potential for thrombocytopenia.
**Option C:** Direct oral anticoagulants (DOACs) are not recommended in pregnancy due to a lack of safety and efficacy data.

**Clinical Pearl / High-Yield Fact**
UFH is typically administered subcutaneously, and its anticoagulant effect is monitored using activated partial thromboplastin time (aPTT) levels.

**Correct Answer:** D. Unfractionated heparin (UFH)