Maryam Farid Mojtahedi - Endocrinology and Female Infertility Unit, Arash Women's Hospital, Tehran University of Medical Sciences, Tehran, Iran.
Saeedeh Aref - Endocrinology and Female Infertility Unit, Arash Women's Hospital, Tehran University of Medical Sciences, Tehran, Iran.
Ashraf Moini - Endocrinology and Female Infertility Unit, Arash Women's Hospital, Tehran University of Medical Sciences, Tehran, Iran. Department of Endocrinology and Female Infertility, Reproductive Biomedicine Research Center, Royan Institute for Reproductive Biomedic
Arezoo Maleki-Hajiagha - Department of Anatomy, School of Medicine, Tehran University of Medical Sciences, Tehran, Iran. Research Development Center, Arash Women's Hospital, Tehran University of Medical Sciences, Tehran, Iran.
Ladan Kashani - Endocrinology and Female Infertility Unit, Arash Women's Hospital, Tehran University of Medical Sciences, Tehran, Iran.

Background: Studies have evaluated different endometrial preparation methods, but the optimal frozen-thawed embryo transfer (FET) cycle strategy in terms of the in-vitro fertilization outcome is still debated. Objective: To compare the natural versus modified natural cycles for endometrial preparation in women undergoing FET. Materials and Methods: This study was designed as a randomized clinical trial, and it was performed at the Arash women’s hospital between August ۲۰۱۶-۲۰۱۸. ۱۴۰ eligible participants were enrolled in this study and were randomly divided into ۲ groups by using the block randomization method, including true natural FET (n = ۷۰) and modified natural FET (mNFET) (n = ۷۰) cycles. Both groups were monitored for endometrial thickness and follicular size; simultaneously spontaneous luteinizing hormone surge using urinary luteinizing hormone testing kits. The mNFET group received ۵۰۰۰ IU of human chorionic gonadotropin injection to trigger final follicular maturation. Luteal support by vaginal progesterone (cyclogest ۴۰۰ mg twice daily) was used in true natural FET from the day of transfer until the ۱۰th wk of pregnancy. Chemical and clinical pregnancy and abortion rates were considered as the primary outcomes. Results: There were no differences in the participants’ baseline characteristics between groups. There was no difference in clinical pregnancy and abortion rate between groups, while the implantation rate was significantly higher in the mNFET group (۲۹.۲% vs. ۱۷.۶%; p = ۰.۰۳۶). Conclusion: The results demonstrated that both types of natural cycles were similar in pregnancy outcomes, while modified cycles might be associated with a higher implantation rate.

Embryo transfer, In vitro fertilization, Pregnancy rate, Live birth, Human chorionic gonadotropin., انتقال جنین, لقاح آزمایشگاهی, میزان بارداری, تولد زنده, گنادوتروپین جفتی انسانی.