BACKGROUND: Borderline serous tumors (BSTs) are epithelial neoplasms of low malignant potential, accounting for approximately 10% to 15% of all ovarian epithelial tumors and are generally associated with a favorable prognosis. Primary fallopian tube BSTs are uncommon and usually detected incidentally. Fallopian tube mesenteric BST arising during pregnancy is extremely rare, with limited literature available. This case highlights the diagnostic challenges of this rare disease and offers practical insights into perioperative management.

CASE REPORT: A 29-year-old woman (gravida 2, para 1) at 37 weeks of gestation with a twin pregnancy underwent elective cesarean delivery. Intraoperatively, a 4×3×3 cm cystic mass originating from the mesentery of the ampulla of the left fallopian tube was identified and excised. The lesion was first detected on ultrasonography 18 months before delivery, but serial imaging during the antepartum period failed to yield a definitive diagnosis. Postoperative histopathological assessment, including gross and microscopic examination, confirmed the diagnosis of BST. Follow-up at 6 and 9 months postoperatively showed no signs of recurrence, and serum CA-125 levels remained within the normal range.

CONCLUSIONS: Fallopian tube mesenteric BST in pregnancy presents substantial diagnostic challenges due to its rarity and non-specific imaging features. Fertility-sparing resection is safe and effective for preserving reproductive function. The complexity of twin gestation requires careful intraoperative decision-making and highlights the value of multidisciplinary teamwork. Thorough intraoperative evaluation and long-term postoperative surveillance are essential to improve long-term outcomes.

Keywords: Case Reports, Fallopian Tube Neoplasms, Cesarean Section, Pregnancy Complications, Neoplastic, CA-125 Antigen