We report a case of metallic prosthetic valve thrombosis in a 24-year-old woman at her 36th week of gestation. With the onset of gestation this patient had been prescribed enoxaparin at a once-daily dose of 0.6 mL, which was lower than the recommended dose. The patient's mitral valve gradient was increased. We gave her intravenous unfractionated heparin because she refused to give consent for thrombolytic therapy owing to its potential hazards for the fetus. After the patient gave birth we treated her with a thrombolytic agent and the thrombus completely dissolved. Pregnancy in patients with mechanical valve prosthesis requires close follow-up and patient adherence to the prescribed medications. In case of thrombus formation both the fetus and the mother are at risk for fatal complications.