Aromatase expression in the endometrium seems to play a pivotal role in the development of endometriotic lesions. Because inflammatory mediators such as prostaglandin E2 appear to activate aromatase in the cells of the endometrial stroma, it was hypothesised that the ensuing inflammation caused by the arrival of aromatase-positive cells in the peritoneal cavity would stimulate local estrogen production, which would in turn facilitate the development of endometriotic lesions by suppressing macrophage phagocytosis. Aromatase expression in the eutopic endometrium will also hamper ovum nidation, thus causing infertility. Progestins, such as gestodene and danazol, are potent inhibitors of aromatase expression in the endometrium, and the use of vaginal rings with danazol in doses that do not block ovulation is associated with the occurrence of pregnancy in patients with severe endometriosis without the need for surgery. A local effect on the endometrium suppressing aromatase expression has been suggested as a possible mechanism of action for the danazol ring.