The discovery of an ovarian mass during pregnancy is often a difficult issue because of the risk related to surgical intervention during pregnancy. Moreover, ultrasound examination is often unable to provide a definitive diagnosis. A case of decidualized ovarian endometriosis is presented to highlight the challenges in this diagnosis. We report the transvaginal ultrasound findings, including color Doppler examination, magnetic resonance characteristics and tumor marker longitudinal evaluation during the first trimester of pregnancy, as well as the final histological characteristics of the lesion. Decidualization is a rare occurrence in ovarian endometriosis and must be differentiated from malignant transformation.