Does hCG-trigger in the mild stimulation protocol for endometrial preparation have any effect on pregnancy outcome in frozen-thawed embryo transfer?

Abstract Background Recent literature has explored the role of human chorionic gonadotropin (hCG) triggering in frozen embryo transfer (FET) cycles with natural endometrial preparation. Despite this, the impact of hCG triggering on pregnancy outcomes following endometrial preparation with mild stimulation (mST) using Letrozole and Gonadotropins remains inadequately characterized. This study aimed to elucidate the effects of hCG-trigger on pregnancy outcomes in mST-FET cycles. Methods In the present retrospective cohort study, the pregnancy outcomes of 409 eligible patients who underwent FET cycles with endometrial preparation using a mild ovarian stimulation protocol by letrozole plus low dose gonadotropins at the Royan Institute between 2020 and 2022, were investigated. The study population were segregated into two distinct groups according to type of ovulation: the spontaneous ovulation group (n = 138) and the hCG-trigger group (n = 271). The pregnancy outcomes including implantation and clinical pregnancy rates (CPR) and live birth rates (LBR) were compared between two groups. The multivariable logistic regression was performed to detect the most significant variables related to the LBR in the mST-FET cycles. Results Demographic and baseline characteristics were comparable between groups. No significant difference was found in terms of implantation rate (0.65 ± 0.32 vs. 0.60 ± 0.30, P-value: 0.31), CPR (37% vs. 39.7%, P-value: 0.53), and LBR (35.5% vs. 37.3%, P-value: 0.74) in the spontaneous ovulation and hCG-trigger groups, respectively. The logistic regression analysis revealed that only the stage of the transferred embryo exhibited a significant relationship with LBR (blastocyst vs. cleavage: odds ratio (OR); 2.33, 95% confidence interval (CI):1.41–3.86, P-value = 0.001). Conclusion Pregnancy outcomes in the mST-FET cycles, including implantation rate, CPR, and LBR are comparable in cycles with or without hCG triggering. Based on the findings from multivariate regression analysis, the sole significant predictive factor for the LBR was the transfer of blastocyst embryos. It is recommended that these results be examined and discussed in future prospective studies with a larger sample size, considering the lack of comparable research in this field.