IRCT201607311556N92 IRCT 2016-08-24 Tehran University of Medical scinces/Vice chancellor for research Comparing the rate of pregnancy of Natural versus modified –natural cycle in frozen embryo transfer Comparing the rate of pregnancy of Natural versus modified –natural cycle for endometrial preparation in frozen embryo transfer 2016-08-22 anticipated 140 Complete https://irct.ir/trial/941 interventional Randomization: Randomized, Blinding: Single blinded, Placebo: Not used, Assignment: Parallel, Purpose: Treatment, Randomization description: The patients underwent FET according to a computer-generated concealed randomization list using the block randomization method. There were six blocks. The statistician prepared random treatment assignments in sealed envelopes to conceal the randomization list from all research staff involved in enrollment and assessment, Blinding description: This study was designed as a single blinded study and the outcome investigators (infertility clinic’s midwife and the gynecologist who performed ultrasound assessments), and the statistician were unaware of the type of intervention until the end of the study. N/A In vitro fertilization. Intervention 1: The control group(1): progestron pessary administration after positive urinary LH kit. Intervention 2: 5000 Iu HCG injection IM state, after presenting endometrial thickness>7 mm and 17 mm dominant follicule in trans-vaginal ultrasonography. Undecided - It is not yet known if there will be a plan to make this available Justification or reason for indecision in sharing IPD is The research team has not yet made a decision on this.