**Core Concept**
Prosthetic valve patients require anticoagulation therapy to prevent valve thrombosis and thromboembolic events. In pregnancy, the risk of valve thrombosis increases due to increased blood volume, cardiac output, and coagulability. Heparin is a preferred anticoagulant in pregnancy due to its safety profile, particularly in the first trimester.

**Why the Correct Answer is Right**
The American Heart Association recommends switching to unfractionated heparin (UFH) in pregnancy, particularly in the first trimester, due to the risk of warfarin embryopathy. Warfarin crosses the placenta and can cause fetal abnormalities, including nasal hypoplasia and stippled epiphyses. Heparin does not cross the placenta and is a safer alternative. However, low molecular weight heparin (LMWH) is also a suitable option in pregnancy due to its predictable pharmacokinetics and reduced risk of bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as warfarin is not recommended in the first trimester of pregnancy due to the risk of fetal abnormalities.
**Option B:** This option is incorrect as the timing of switching to heparin is not specified, and it does not provide a clear recommendation.
**Option C:** This option is incorrect as it does not specify the type of heparin to be used (UFH or LMWH).

**Clinical Pearl / High-Yield Fact**
In patients with mechanical heart valves, the American Heart Association recommends using warfarin for the first 28 days postpartum to prevent valve thrombosis and thromboembolic events.

**Correct Answer:** B. During the first trimester of pregnancy.