It is important to note: there is no perfect anticoagulation regimen during pregnancy in patients with mechanical valve replacement, and warfarin and heparin each have their pros and cons. The 2014 US ACC/AHA Guidelines for the Management of Valve Disease recommend that Class I: patients with prosthetic mechanical valves should be anticoagulated with warfarin in mid- to late pregnancy and at target. Warfarin is the most effective anticoagulant. Although warfarin is teratogenic in early pregnancy, it has little or no effect in mid to late pregnancy. Normal heparin anticoagulation increases the risk of maternal death. The risk of thromboembolism with warfarin throughout pregnancy is <4%, compared with a 33% risk with regular heparin throughout pregnancy. Fatal prosthetic valve embolism has been reported with unmonitored low-molecular heparin anticoagulation, and if anti-Xa levels are monitored, the embolic complications of low-molecular heparin anticoagulation are lower than those of plain heparin. However, prosthetic valve embolism has been reported with full application of low molecular heparin even when anti-Xa levels are monitored. Warfarin is safer for pregnant women but has fetal effects. Heparin is safe for the fetus but has implications for the pregnant woman because of its weaker anticoagulant effect. Class IIa: If a pregnant woman with a prosthetic mechanical valve requires ≤5 mg of warfarin per day to achieve her anticoagulation goal, warfarin anticoagulation may be continued after adequate weighing of the pros and cons with the pregnant woman. The teratogenic effects of warfarin are dose-dependent, with a low risk of teratogenesis (<3%) at doses less than 5 mg daily. Therefore, warfarin anticoagulation can be continued after full consultation with the pregnant woman to better protect her Class IIa: If the pregnant woman with a prosthetic mechanical valve requires warfarin >5 mg daily, it is reasonable to administer the appropriate dose of low molecular heparin twice daily with adequate monitoring of anti-Xa levels (0.8 U/mL to 1.2 U/mL, 4-6 hours post-dose). If warfarin is >5 mg daily, the teratogenic risk is >8%. Heparin does not cross the placenta and can be an alternative to anticoagulation. The dose of low molecular heparin is adjusted according to the level of anti-Xa, since the required dose of low molecular heparin may increase by 50% during pregnancy. There are some controversial issues regarding the use of LMH, including the optimal level of anti-Xa, the time of monitoring (peak or trough concentration), and whether to administer LMH twice or three times daily.