[Ectopic decidua: variability of presentation in pregnancy and differential diagnoses]

The ectopic decidua is a common occurrence during pregnancy, corresponding to the presence of decidual tissue outside the endometrium. The pathophysiology of these lesions is not yet fully elucidated. The phenomenon is most often benign, with a favourable outcome in the post-partum and without follow-up. Because of the wide range of clinical presentations, from localized peritoneal granulation to large pelvic mass, the clinical diagnosis of ectopic decidua is difficult. Furthermore, imaging is not specific. The discovery of such lesions should lead to the realization of systematic biopsies for histological examination and rule out other causes such as lesions of tuberculosis or carcinomatosis. Immunohistochemical studies are helpful in differentiating deciduosis from malignant deciduoid mesothelioma, its main differential diagnosis with a catastrophic prognosis.