A prosthetic valve patient switch to heparin at which time of pregnancy:
**Core Concept**
Warfarin and heparin are anticoagulants used in patients with prosthetic valves. Warfarin crosses the placenta and can cause fetal abnormalities, whereas heparin does not cross the placenta but has a higher risk of bleeding. The decision to switch from warfarin to heparin during pregnancy is crucial for the health of both the mother and the fetus.
**Why the Correct Answer is Right**
During the first trimester of pregnancy, warfarin is avoided due to its teratogenic effects. The American Heart Association recommends switching from warfarin to heparin between 28-32 weeks of gestation to minimize the risk of fetal abnormalities. This is because warfarin can cause fetal warfarin embryopathy, which includes nasal hypoplasia, limb abnormalities, and stippling of the bones. Heparin, on the other hand, does not cross the placenta and is safer for the fetus. However, it requires more frequent monitoring to prevent maternal bleeding.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because switching to heparin too early in pregnancy may not provide adequate protection against fetal warfarin embryopathy.
**Option B:** This option is incorrect because switching to heparin too late in pregnancy may increase the risk of maternal bleeding and fetal warfarin embryopathy.
**Option C:** This option is incorrect because the timing of 20-24 weeks is too early for switching to heparin, increasing the risk of fetal warfarin embryopathy.
**Clinical Pearl / High-Yield Fact**
The American Heart Association recommends switching from warfarin to heparin between 28-32 weeks of gestation in patients with prosthetic valves to minimize the risk of fetal abnormalities.
**Correct Answer:** C.