Introduction: Ovarian cysts are uncommon in live-born female newborns, with a frequency of roughly 1: 2500. Few occurrences of serous cystadenoma have been recorded, however, increasing gestational age, maternal diabetes, preeclampsia, and rhesus isoimmunization may increase the frequency of ovarian cysts. Case report: A 3-month-old girl born 34 weeks prematurely and complaining of increased abdominal distension was brought by her mother. On abdominal computed tomography, a large unilocular ovarian cyst occupied the whole abdomen. Laparoscopic assessment indicated a pelvic ovarian cyst mass containing 1000 ml and gross examination revealed a unilocular ovarian cyst.