Ectopic decidualization, or deciduosis, is a benign, hormone-dependent proliferation of decidual-type stromal cells outside the uterine endometrium. During pregnancy, it is most often an incidental finding. Yet, its nodular gross appearance and cellular histology can closely imitate disseminated malignancy, exposing patients to overtreatment when it is misread intra-operatively or on biopsy. A 34-year-old second-gravida woman was admitted in active labour and had an uncomplicated full-term vaginal delivery. On the second postpartum day, during elective tubal ligation, a 5 × 4.5 × 3 cm right ovarian cyst studded with off-white, shiny surface nodules was identified and excised together with bilateral tubal ligation. Histology demonstrated a serous cystadenoma lined by a single layer of flattened-to-cuboidal epithelium, accompanied by surface and parenchymal nodules of bland, large polygonal cells with abundant eosinophilic cytoplasm consistent with decidua. Immunohistochemistry (PR positive, CD10 positive, focal inhibin positivity, CK20 negative) together with the absence of nuclear atypia or mitoses supported a benign decidual reaction. A systematic search excluded occult malignancy; after multidisciplinary consensus, no further treatment was given, and the patient remained asymptomatic. We situate this case within a medico-informatics framework, proposing that whole-slide imaging with computer-aided image analysis can reduce the diagnostic-error risk posed by benign mimics while generating reusable, machine-readable evidence for rare entities. Clinician and pathologist awareness of the condition, stringent histological criteria, and digital diagnostic infrastructure together protect pregnant patients from unnecessary intervention.