Vanishing pelvic mass: Decidualized endometriosis during pregnancy.

A 40-year-old woman without history of endometriosis was found to have 10 cm pelvic mass on the routine first trimester ultrasound. Magnetic resonance imaging (MRI) of the pelvis demonstrated a large solid mass abutting the rectum which raised the concern for malignancy. Transrectal biopsy of the mass was performed with histopathology result of decidualized endometriosis. Patient continued her pregnancy and had cesarean section at 39 weeks. Interestingly, no mass was found when obstetrician performed pelvic examination after delivery in the operative room. This case is a unique presentation of endometriosis during pregnancy in a patient with no prior history of endometriosis. Large size and abutment of the rectum by the decidualized endometriosis on MRI led to misinterpretation as malignancy. Our case highlights complexity of the deep infiltrative endometriosis (DIE) during pregnancy which can misguide the providers, lead to unnecessary procedures and unwanted complications.