Objective: To investigate the impact of regressed endometrial hyperplasia (EH) on pregnancy, obstetric and neonatal outcomes following frozen embryo transfer (FET) after conservative treatment. Design: Retrospective cohort Setting: Reproductive center, Shanghai Jiaotong University School of medicine, Renji Hospital Population: 95 patients with EH regression after conservative treatment and 30634 infertile controls with presumed normal endometrium, fom January 2015 to December 2022. Methods: Medical records from our database were analyzed. Propensity score matching, performed at a ratio of 1:4 between EH and control patients, and binary logistic regression were applied to adjust for potential confounding factors. Main Outcome Measures: The primary outcome was live birth rate. Results: Compared with matched controls, patients with EH exhibited a significantly lower live birth rate (31.6% vs. 49.2%, P = 0.002), reduced clinical pregnancy rate (44.2% vs. 59.2%, P = 0.008), and a higher incidence of pregnancy loss (40% vs. 26.1%, P = 0.046). Additionally, cervical insufficiency occurred more frequently in EH patients (6.7% vs. 0%, P = 0.019). No significant differences in these outcomes were observed between EH patients with and without atypia. Furthermore, logistic regression analysis confirmed that EH was an independent risk of live birth following FET (adjusted OR 0.63, 95% CI: 0.41-0.95, P = 0.04). Conclusions: This large-scale study showed that even achieving completed remission, EH continued to adversely impact live birth following FET cycles. These finding highlight the importance of thorough conception counseling and intensive pregnancy monitoring in this high-risk population.