Objective: Ectopic decidual reaction (deciduosis) can be seen rarely on the peritoneum during laparotomy for a cesarean section for pregnancy, in addition to the ovary and cervix. The aim of this study was to evaluate the clinical, histopathological, immunohistochemical characteristics of ectopic decidua cases that were incidentally found in the peritoneum during a cesarean section.

Material and Method: A total of seven cases where decidualization was found in the peritoneal biopsy taken during pregnancy at the Baskent University Medical Faculty Adana Teaching and Training Hospital Department of Pathology were included in this study. The clinical features of the cases were obtained from their clinical folders. The morphological findings were recorded and the peritoneal biopsies were analyzed with keratin Pan Ab-1, calretinin, vimentin, HMB-45 and progesterone receptor antibody for immunohistochemical analysis.

Results: The mean age for the seven cases was 36±4.16. The gestational age was 33 to 39 (mean 37.2) weeks. Microscopic evaluation revealed decidualized cells that were large polygonal and eosinophilic, some with vacuolated cytoplasm, that formed small nodules under the mesothelium of the peritoneum in all cases. Immunohistochemical staining showed positive staining of the cell cytoplasm with vimentin and positive staining of the cell nucleus with the progesterone receptor antibody in the decidual cells. Calretinin, keratin and HMB-45 stains were negative.

Conclusion: Pregnancy-related peritoneal deciduosis develops with the effect of progesterone in pregnancy. It disappears without complication in the postpartum period. Immunohistochemistry may help the differential diagnosis of peritoneal deciduosis where problems are experienced differentiating the case from malignant mesothelioma or metastatic tumor.