**Core Concept**
The management of pregnant patients with prosthetic valves requires careful consideration of anticoagulation therapy to prevent valve thrombosis and embolic events. **Warfarin**, a commonly used anticoagulant, has teratogenic effects, particularly during the first trimester. **Heparin** is often considered as an alternative due to its safer profile during pregnancy.

**Why the Correct Answer is Right**
The correct answer is related to the timing of switching from **warfarin** to **heparin** to minimize the risk of teratogenic effects. This switch is typically recommended at the beginning of the pregnancy, specifically around 6-12 weeks of gestation, to avoid the risks associated with **warfarin** use during organogenesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because switching too early may not be necessary if the patient is not yet pregnant.
**Option B:** This option is incorrect because it may be too late to switch, given the potential for **warfarin**-induced teratogenicity.
**Option D:** This option is incorrect as it is beyond the period when the switch would be most beneficial in preventing teratogenic effects.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the management of anticoagulation in pregnant women with prosthetic valves is complex and requires close monitoring. The switch from **warfarin** to **heparin** should be tailored to the individual patient's risk factors and the gestational age.

**Correct Answer:** Correct Answer: B. 36 weeks is not correct, the actual answer is that the switch should occur at around the 6th to 12th week to avoid teratogenic effects, which corresponds to none of the given options, but typically the switch would be considered around the beginning of the pregnancy to minimize risks.