Ovarian malignancy is the second most common gynaecological malignancy diagnosed during pregnancy. A grand multipara, aged 30 years presented with gestational amenorrhea with abdominal discomfort and breathlessness for last 15 days. Sonographic examination demonstrated a huge, unilocular ovarian cyst and an alive fetus of about 30 weeks gestation. Intraoperative findings were huge left ovarian cyst (42x40x20 cm) with straw coloured mucinous fluid. Left salpingo-oophorectomy was performed followed by peritoneal washings and omental biopsy. Histopathology revealed mucinous cystadenomas with inflammatory changes in omentum and no malignant cells in peritoneal washings. She delivered vaginally a female baby of 3.5 kg at 38 weeks with good Apgar score.