The aim of our study is to evaluate the efficacy of dienogest versus GnRH-a in improving ongoing pregnancy rates in women undergoing IVF due to endometriosis. We will conduct a non-blinded randomized controlled trial. One group will receive dienogest 2mg daily for a period of 3 months followed by a standard IVF/Intracytoplasmic Sperm Injection (ICSI) cycle. The second group will receive one injection of 3.75mg of GnRH-a every 28 days for three doses followed by a standard IVF/ICSI cycle 3 months later. The third group will not receive any medical interventions before the planned IVF/ICSI cycle. We hypothesize that patients receiving dienogest will have similar ongoing pregnancy rates compared to patients receiving the GnRH-a injection. Secondary outcomes including number of gonadotropins consumed, number of stimulation days, number of metaphase II eggs retrieved, fertilization rate, embryo quality, miscarriage rate, clinical pregnancy rates, live birth rates and potential maternal and obstetrical complications will also be evaluated. We will also compare ongoing pregnancy rates between the groups receiving Dienogest and placebo, and GnRH agonist and placebo.