A 28-year-old primigravida female presented at 12 gestational weeks to the Emergency Department of Minia University Hospital, ElMinia, Egypt, in 2017 with severe dyspnoea upon exertion. Five years earlier, she had undergone mitral valve replacement with a mechanical prosthesis. After her pregnancy had been confirmed, she had discontinued anticoagulant warfarin therapy before taking low-molecular weight heparin. There was no significant history of medical diseases or prescription of any other medications.Transoesophageal echocardiography (TEE) revealed a large thrombus of approximately 4 cm2 on the mitral prosthesis, with near total obstruction of the prosthetic mitral valve. An abdominal ultrasound revealed that the fetus was non-viable. Three days after admission, the patient underwent surgery. This revealed a large thrombus on the atrial aspect of the mitral prosthesis projected away from the valve disc. Both the prosthetic valve and the entire thrombus were removed surgically and another valve was inserted. An obstetrician performed a dilation and evacuation procedure on the fifth day after the open-heart surgery. At a six-month follow-up, the patient had recovered well and was healthy.