Background: To explore the predictive value of endometrial receptivity and pregnancy outcomes using pipelle biopsy examination during the luteal phase of the menstruation cycle in infertile women. We also compare the importance of this factor with other potential causes of infertility. Methods: This retrospective study included 279 women with repeated implantation failures. All patients were examined and treated at the Intersono in vitro fertilization (IVF) center. To determine the cause of the implantation failure, an implantation window (IW) was defined for all patients included in this retrospective cohort. Examinations were performed by three-fold aspiration biopsies of the endometrium during the luteal phase of the cycle as this period is when the endometrium is most receptive for implantation. Analysis of the tissue was done by scanning electron microscopy of endometrial tissues and, based on results obtained, an endometrial preparation protocol was individualized for the next attempt. Statistical analyses were performed using Microsoft Excel (Office 365) and RStudio software v. 1.4.1106. Results: According to the results obtained, 206 women included in this study displayed a unique IW (74%), and 73 women had a standard IW period (26%). Patient characteristics, screening indicators, previous IVF treatment details, IVF cycle characteristics, as well as number, quality, and stage of embryos transferred were comparable between the two groups. Clinical pregnancy rates of 59.2% (95% confidence interval (CI) 52.5–65.8) versus 57.5% (95% CI 46.1–68.6) (p = 0.80), and live birth rates of 50.7% (95% CI 43.9–57.6) versus 47.9% (95% CI 36.6–59.4) (p = 0.49) in the unique and standard IW groups, respectively, did not show significant differences. Conclusions: Unique IW is one of the underlying causes of implantation failures. The personalization of an endometrial preparation protocol is a method to improve IVF outcomes.