A cystic pelvic mass detected during pregnancy is not uncommon, but can be a diagnostic challenge. Most of these pelvic masses are benign ovarian cysts and resolve spontaneously. However, rare non-ovarian causes can complicate the diagnostic and therapeutic management. In this report an unusual case is presented of a 27-year-old pregnant woman with an atypical multicystic lesion in the pelvis, seen on routine first trimester ultrasound. A laparoscopic exploration was performed to rule out ovarian malignancy, and after histological analysis she was diagnosed with peritoneal inclusion cysts. The diagnostic and therapeutic challenges inherent to this rare non-ovarian disease are addressed in this case report.