The assessment of demographic features, follicular phase parameters, and luteal support in mild stimulation frozen embryo transfer cycles for optimal ongoing pregnancy outcome

Objective We aimed to identify independent predictors of ongoing pregnancy in patients undergoing mild-OS FET cycles, focusing on follicular phase characteristics and luteal support regimens.Methods In this multicenter, retrospective cohort study conducted between January 2021 and August 2024 across Bahceci in vitro fertilization (IVF) centers in Türkiye, 489 FET cycles with mild-OS using letrozole were analyzed. Biochemical and ongoing pregnancy outcomes were assessed in relation to demographic characteristics, ovarian stimulation parameters, ovulation triggering strategies, and LPS approaches. Multivariate logistic regression was used to determine independent predictors.Results The overall biochemical and ongoing pregnancy rates were 58.5% and 43.4%, respectively. Subcutaneous progesterone (with or without vaginal route) improved biochemical pregnancy rates compared to vaginal-only LPS (67.1% vs. 52.7%, p = 0.008), although this did not translate into significantly higher ongoing pregnancy rates. In adjusted models, only younger female age (OR = 0.94, 95% CI: 0.90 to 0.99, p = 0.01) and a higher number of embryos transferred (OR = 2.00, 95% CI: 1.16 to 3.45, p = 0.01) were independently associated with ongoing pregnancy. Follicular diameter, number of follicles >10 mm, estradiol and LH levels, or triggering with hCG did not significantly impact outcomes.Conclusion In mild-OS FET cycles, while subcutaneous progesterone support may improve early implantation outcomes, ongoing pregnancy is primarily influenced by female age and embryo number. These findings support a flexible approach to LPS and triggering in mild-OS protocols without compromising clinical success.