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Managing women with mechanical heart valves during pregnancy poses a particular challenge as there are no available controlled clinical trials to provide guidelines for effective antithrombotic therapy. Oral anticoagulation with coumadin-derivates administration is associated with coumarin embryopathy, and subcutaneous administration of unfractioned heparins (UFH) has been reported to be ineffective in preventing thromboembolic complications. Due to the increased risk of thromboembolic events when UFH is used, low molecular weight heparins (LMWH) were considered to be an alternative. The evidence in the literature regarding the long-term use of LMWH as the only anticoagulant after mechanical heart valve replacement is limited only to a few reports encompassing only 25 patients, with treatment failure in 20%. These data show that anticoagulation with LMWH only is neither safe nor effective in preventing thromboembolic events after mechanical heart valve replacement, in pregnant or non-pregnant women.