Anticoagulant choices in pregnant women with mechanical heart valves: balancing maternal and fetal risks--the difference the dose makes.

Long-term anticoagulation is required in all patients with mechanical prosthetic heart valves to prevent complications with valve thrombosis and valve failure or systemic thromboembolism. The prothrombotic environment of pregnancy further increases the risks of these complications. Anticoagulant choices for pregnant women include oral vitamin K antagonists such as warfarin, or heparin - either unfractionated heparin (UFH) or low molecular weight heparin (LMWH). None of the options is without risk for the mother or her baby. Warfarin crosses the placenta and is associated with warfarin embryopathy and fetopathy but is very effective at preventing thromboembolic complications. The dose of warfarin may play a role in the risk of some, but not all fetal complications. Heparin does not cross the placenta but is less effective at preventing thrombosis and LMWH may be more effective than UFH. The optimal dose and target anti-Xa levels for LMWH have not been established. Measurement of trough anti-Xa levels in addition to peak anti-Xa levels may be important.