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Background: Progestin therapy is an option for patients with endometrial carcinoma (EC) or endometrial intraepithelial neoplasm (EIN) who fit specific criteria of fertility-sparing treatment. However, the implantation rate remains low among females receiving in vitro fertilization (IVF) even after the complete reversal of endometrial lesions. Methods: Here, ten patients with EC/EIN achieved complete regression (CR) in histology. Their relevant metabolic and IVF parameters were collected. An endometrial sampling at the window of implantation (WOI) and transcriptome analysis were conducted among them, and four healthy controls were analyzed to analyze endometrial receptivity. Results: On average, it took ten patients five months to achieve CR after four curettage procedures. The interquartile range of endometrium thickness on trigger day was between 8.8 and 10.0 mm, while the range was 15.2–18.5 mm for controls. Five patients got pregnant after a frozen-embryo transfer. According to ERA analysis, the endometrial sampling at WOI showed pre-receptive status in four cases. In total, 1458 differential expression genes were identified, and 70 belonged to the ERA genes. ImmuneScore indicated decreased NK cells in the endometrium, affecting endometrial receptivity. Conclusions: Even after EC/EIN reversal in histology, endometrial receptivity has already been compromised regarding altered WOI and immune microenvironment, leading to a low pregnancy rate.