Natural cycle versus modified natural cycle for endometrial preparation in women undergoing frozen-thawed embryo transfer: An RCT.

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 2016-2018. Hundred and forty eligible participants were enrolled in this study and were randomly divided into 2 groups by using the block randomization method, including true natural FET (n = 70) and modified natural FET (mNFET) (n = 70) 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 5000 IU of human chorionic gonadotropin injection to trigger final follicular maturation. Luteal support by vaginal progesterone (cyclogest 400 mg twice daily) was used in true natural FET from the day of transfer until the 10 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 (29.2% vs. 17.6%; p = 0.036). 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.