Ectopic decidua (deciduosis) is most commonly localized in the ovary, uterus, cervix, and tuba uterina. It can rarely be observed within the peritoneum in pregnant women during laparotomy. More rarely, it can be localized in the omentum, appendix, liver, and spleen. It is usually incidental. In a 23-year-old female patient, a biopsy was taken from the thickening in a 4x3 cm area on the omentum during cesarean section. Microscopic evaluation revealed decidualized cells the majority of which had large polygonal eosinophilic cytoplasm and a few of which had vacuolated cytoplasm, that formed small nodules in the adipose tissue. Immunohistochemically, the decidualized cells were positive for vimentin, progesterone receptor antibody and negative for S-100, HMB-45, calretinin, pancytokeratin. The case was reported as ectopic omental deciduosis. Although ectopic omental deciduosis is a benign lesion, it may be confused with malignant tumors. Therefore, differential diagnosis should be made carefully.
Key words: Omental deciduosis, ectopic decidua, pregnancy, progesterone