Trial Comparing Preoperative Dienogest Therapy Followed by Surgery vs. Upfront Surgery to Save Ovarian Reserve in Young Women With Ovarian Endometrioma

Inclusion Criteria: * Unilateral ovarian endometrioma ≥ 5cm or bilateral ovarian endometriomas with any size diagnosed by ultrasonography. * In Patients with previous unilateral salpingo-oophorectomy (USO) * Unilateral ovarian endometrioma with any size is eligible * 20 ≤ Age ≤ 45 and premenopause * Plan to undergo conservative surgery for endometriomas Exclusion Criteria: * Pregnant women or women who were suspected to be pregnant * Women with current venous thromboembolism or history of such diseases * Women with current arterial disease or cardiovascular diseases (e.g. myocardial infarction, cerebrovascular diseases, or ischemic heart diseases) or history of such diseases * Women with current diabetes with vascular lesions or history of such diseases * Women with current severe liver diseases or history of severe liver diseases with abnormal liver function * Women with current liver tumor or history of liver tumor * Women with current or history of sex-hormone dependent malignant tumor or women who were suspected to have a sex-hormone dependent malignant tumor * Women with vaginal bleeding of unknown causes * Women with a history of allergic reaction to elements of DNG * Women with galactose intolerance, Lapp lactase deficiency, glucose-galactose malabsorption * Women whose non-compliance is expected * Lactation (treatment with DNG is not recommended in lactating women due to possible excretion in breast milk) * Any medication that could result in an excessive accumulation, impaired metabolism, or altered excretion of the study drug or might interfere with the conduct of the study or the interpretation of the results