IRCT201306256689N3 IRCT 2013-12-31 Vice Chancellor for Research, Tehran University of Medical Sciences Comparing two protocols for endometrial preparation for frozen embryo transfer Comparing pregnancy rate in GnRH agonist and letrozole plus gonadotropins protocol for endometrial preparation for frozen embryo transfer 2012-08-26 anticipated 100 Complete https://irct.ir/trial/7100 interventional Randomization: Randomized, Blinding: Not blinded, Placebo: Not used, Assignment: Parallel, Purpose: Treatment, Other design features: All patients sign informed consents forms. The sample included 100 patients who are divided in two groups randomly based in Bernoli distribution. 2 Infertility. Intervention 1: First group receives GnRH Agonist (Bucerelin, Aventis, Germany) 500 μg subcutaneously from previous midluteal cycle (21th day). Then Estradiol valerat (Daroopakhsh, Iran, 2 mg) daily oral is started on second day and is increased until the observation of 8mm endometrial thickness in transvaginal ultrasound. For visualizing endometrial thickness, Trans vaginal ultrasound (Sonoline G20; Siemens Medical Solutions, California, USA) is performed every 4 days. After the observation of at least 8mm endometrial thickness, Progesterone (Cyclogest, Germany, 800 mg) is started vaginally. After 3 days, Trans cervical embryo transfer will be carried out on day 16 to 19. Intervention 2: Second group, receives letrozole (Iranhormone, Iran, 5mg daily) orally on 2th day of cycle for five days. Then HMG (Ferring, Germany, 75IU daily) is started on 7 day. After the observation of 18mm follicle in transvaginal ultrasound, HCG (Ferring, Germany, 10000IU, and IM) is injected for ovulation induction. Trans cervical embryo transfer is performed on day 16 to 19.