LAUSR

Background During pregnancy, the discovery of adnexal masses remains frequent. Such masses are mostly benign. Ovarian endometrioma is a rare etiology. The diagnosis may be difficult in some situations, such as decidualization. It may be asymptomatic or result in complications for which magnetic resonance imaging is needed. Case presentation We describe an unusual case of decidualization of an ovarian endometrioma complicated by a sigmoid fistula during a 7-week, 1-day pregnancy in a Arabic patient aged 38 years who developed acute pelvic pain with fever. She had a medical history of unexplored secondary dysmenorrhea. The diagnosis was suspected on the basis of magnetic resonance imaging findings. The management was based on surgery, during which exploration revealed a mass at the expense of the left ovary being very adherent and fistulized to the sigmoid. We performed adnexectomy followed by digestive ostomy. The result of pathological study with immunohistochemistry led to a diagnosis of decidualization of an ovarian endometrioma altered by infection. Conclusion Decidualization of an ovarian endometrioma can lead sometimes to unexpected complications. The decision to provide surgery must be made with caution without delaying treatment in the event of a deep suspicion of malignancy and/or complication. The particular and exceptional complication discovered in our patient is the fistulization to the sigmoid.