We present the case of a woman in her 20s with an eight-month history of increasing abdominal distention, dyspnea, and night sweats. The patient believed she was pregnant despite being told at another hospital that the pregnancy tests were negative, and no fetus was seen on an abdominal ultrasound. The patient delayed obtaining follow-up because of a distrust of the healthcare system and presented to our hospital at the behest of her mother. On physical examination, the abdomen was distended with a positive fluid wave, and a large mass was palpated in the abdomen. Gynecological examination was limited because of severe abdominal distension but a mass was palpable in the right adnexa. A pregnancy test and fetal ultrasound were performed, and the patient was not pregnant. A CT scan of the abdomen and pelvis revealed a large mass arising from the right adnexa. She underwent right salpingo-oophorectomy, appendectomy, omentectomy, lymph node dissection, and peritoneal implant resection. The biopsy confirmed intestinal-type IIB primary ovarian mucinous adenocarcinoma, expansile type, with peritoneal spread. Chemotherapy was provided for three cycles. A follow-up CT scan of the abdomen showed no evidence of a tumor six months after surgery.