Pregnancy is associated with an increased risk of valve thrombosis, hemorrhagic complications, and offspring complications in patients with metallic prosthetic heart valve (MHV). Warfarin treatment is the best regimen against thromboembolic complications, but its use in the first trimester can result in embryopathy. Low molecular weight heparin (LMWH) does not cross the placenta and has some potential advantages. However, the pharmacokinetics of LMWHs change during pregnancy, and serial monitoring of anti-Xa levels is strongly recommended. Despite this recommendation, LMWH therapy in a fixed dose is still used in pregnant women with MHV without monitoring anti-Xa activity in clinical practice. We present three cases of MHV thrombosis occurring while on therapy with LMWH during pregnancy. One of these patients showed cerebrovascular event, one presented with pulmonary edema, and one underwent reoperation for MHV thrombosis.