**Question:** A patient at 22 weeks pregnancy is on warfarin for oral anticoagulation for a known cardiac condition. Heparin is re-introduced at what gestation?

**Core Concept:** Warfarin and heparin are anticoagulants with distinct mechanisms of action. Warfarin is a vitamin K antagonist, while heparin is a direct thrombin inhibitor. In pregnant patients, the decision to switch from warfarin to heparin depends on the gestational age to minimize the risk of bleeding complications.

**Why the Correct Answer is Right:** Heparin is preferred over warfarin in late pregnancy due to its shorter half-life, rapid onset of action, and reduced risk of neonatal bleeding complications. The decision to switch to heparin is based on the risk of maternal bleeding outweighing the risk of thrombosis and fetal bleeding.

**Why Each Wrong Option is Incorrect:**

A. 20 weeks: Although a reasonable gestational age, it may not be considered safe enough to switch from warfarin to heparin due to the increased risk of bleeding complications in the fetus at this stage.

B. 24 weeks: While later than the optimal time to switch, this option is still too early compared to the optimal gestational age range.

C. 36 weeks: This is too late, as the risk of maternal bleeding complications may outweigh the risk of thrombosis at this stage.

D. 38 weeks: Similar to option C, this is too late, as the risk of maternal bleeding complications may still outweigh the risk of thrombosis in late pregnancy.

**Clinical Pearl:** Heparin should be considered for anticoagulation in pregnant patients with a high risk of thrombosis after 34 weeks of gestation. In this case, the patient is receiving warfarin for a cardiac condition, making heparin a more appropriate choice.

**Correct Answer:** C. 34 weeks: Heparin is considered for anticoagulation in pregnant patients after 34 weeks of gestation, which is when the risk of maternal bleeding complications may be deemed to outweigh the risk of thrombosis. This decision is based on a careful assessment of individual patient factors, thrombosis risk, and fetal bleeding risk.