**Core Concept**
Low molecular weight heparins and unfractionated heparin are used to prevent thromboembolic events in pregnant women with mechanical heart valves. The timing of anticoagulation initiation is crucial to balance the risk of maternal thromboembolism against fetal complications.

**Why the Correct Answer is Right**
The American College of Cardiology and American Heart Association recommend the initiation of anticoagulation therapy in pregnant women with mechanical heart valves at 6-12 weeks of gestation. This timing allows for the establishment of a therapeutic anticoagulant effect while minimizing the risk of fetal complications, such as birth defects and growth restriction. The choice of unfractionated heparin (UFH) over low molecular weight heparins (LMWH) is often based on the presence of mechanical heart valves, as UFH can be more easily reversed in case of an emergency.

**Why Each Wrong Option is Incorrect**
**Option A:** 24 weeks - This option is incorrect because initiating anticoagulation at 24 weeks may not provide adequate protection against thromboembolic events, which can occur earlier in pregnancy.
**Option B:** 36 weeks - This option is incorrect because initiating anticoagulation at 36 weeks may not allow sufficient time to establish a therapeutic effect, increasing the risk of maternal thromboembolism.
**Option C:** Postpartum - This option is incorrect because postpartum anticoagulation may not prevent thromboembolic events that occur during pregnancy.

**Clinical Pearl / High-Yield Fact**
The timing of anticoagulation initiation in pregnant women with mechanical heart valves can be remembered using the acronym "6-12 weeks" to ensure optimal prevention of thromboembolic events while minimizing fetal complications.

**Correct Answer: C. 6-12 weeks.**