IRCT2017081335670N1 IRCT 2017-10-08 Vice chancellor for research of Iran University of Medical Sciences Comparing the rate of pregnancy of Natural cycle and Hormonal cycle in frozen embryo transfer Comparison of pregnancy rate for the best method for endometrial preparation in natural and hormonal cycles for the transfer of freeze-thawed embryos in infertile patients 2016-03-19 anticipated 200 Complete https://irct.ir/trial/26891 interventional Randomization: Randomized, Blinding: Double blinded, Placebo: Not used, Assignment: Parallel, Purpose: Treatment, Other design features: Randomization is based on close envelope method. N/A Infertility. Intervention 1: Control group: Natural cycles with 100 infertile patients will be studied based on the daily serum LH of the 10th cycle day. When the serum LH level will be decreasing, ovulation will occur 36 to 40 hours later. The embryo transfer will be scheduled 3 to 5 days after ovulation. Intervention 2: Intervention group:. In hormonal cycles, 4 to 6 mg of Estradiol Valerate tablet will be taken from day 3 of the cycle. This group on 12th cycle day will undergo transvaginal sonography (TVS) for evaluation of endometrial thickness and if at least endometrial thickness 8 mm observed and embryos 3 to 5 days later will be transferred.