This report describes a case of spontaneous ovarian hyperstimulation syndrome (OHSS) occurring in a pregnant woman carrying the factor V Leiden mutation. Even though prophylactic treatment for thrombo-embolism was adopted by administering low molecular weight heparin, the pregnancy was complicated by thromboses of the left subclavian, axillary, humeral and internal jugular veins during the second trimester of gestation. The pregnancy was managed conservatively and a healthy newborn was delivered at term. In order to avoid unnecessary laparotomy, we emphasize the importance of careful diagnosis in order to differentiate spontaneous OHSS from ovarian carcinoma, as well as the necessity to look for the presence of coagulation disorders in women affected by OHSS.