Does the recovery of serum FSH influence the ovarian sensitivity index in the follicular-phase depot GnRH agonist protocol?

Abstract Background The follicular-phase long GnRHa protocol is a common protocol in COH. This study mainly analyzed the recovery of FSH levels on the day of Gn initiation compared with basal FSH levels on the day of downregulation and studied the correlation with the OSI to discover a new monitoring target for COH. Methods A total of 2286 patients who underwent IVF/ICSI with the follicular-phase depot GnRH agonist protocol from January 2018 to January 2023 were stratified into cohorts. The primary outcome was the OSI in patients who received the follicular-phase depot GnRH agonist protocol. The GnFSH/bFSH ratio was used to represent FSH recovery. The enrolled patients were divided into 3 groups according to GnFSH/bFSH tertiles, and cycle characteristics and outcomes were compared. The relationship between the GnFSH/bFSH ratio and OSI was analyzed using smoothed curve fitting to determine the threshold or saturation point for the study population. Then, the relationship between the GnFSH/bFSH and OSI was tested with a multivariable linear regression model after adjusting for potential confounders. Result(s) After adjusting for potential confounders, when the GnFSH/bFSH ratio was less than 0.8, the OSI increased with increasing GnFSH/bFSH values (β = 2.96, 95% CI = 2.37, 3.56, P < 0.0001). However, there was no significant relationship between GnFSH/bFSH and OSI values when the ratio was higher than 0.8 (β = 0.19, 95% CI= -2.02, 2.39, P = 0.8686). In the multiple linear regression model, after full adjustment, each one-unit increase in the GnFSH/bFSH value was related to increased OSI values (β = 2.59, 95% CI 2.10, 3.09, P < 0.0001). In GnFSH/bFSH tertile groups, patients in T2 (0.37–0.56) and T3 (> 0.56) had increased OSI values (β = 0.49, 95% CI 0.24, 0.74, P = 0.0001 and β = 1.25, 95% CI 0.99, 1.51, P < 0.0001, respectively) when compared with patients with values in T1 (< 0.37). Conclusion(s) The recovery of serum FSH (GnFSH/bFSH) after pituitary suppression is an independent risk factor for increased OSI values in women undergoing the follicular-phase depot GnRH agonist protocol.