Fresh versus frozen embryo transfer in endometriosis-Systematic review v1

Background: Endometriosis is an estrogen-dependent inflammatory disorder that is typically related to infertility, for which assisted reproductive technologies are needed. Fresh embryo transfer can result in hormone levels that are supraphysiological, and this can have an impact on receptivity. Alternatively, frozen embryo transfer can be performed and completed in a separate cycle, but it is unknown if there is a comparable success rate for conception. Methods: The search in PubMed, Europe PMC, and Google Scholar will be done comprehensively using the keywords "endometriosis," "fresh versus frozen embryo transfer," and "fertility outcomes" for articles published from 2010 until January 2026. Only those studies will be included that provide information specifically on endometriosis-patients. Data will be extracted by two authors using predefined criteria. Despite widespread use of IVF in this population, optimal embryo transfer strategies remain controversial. Traditionally, embryos have been transferred in the same cycle as ovarian stimulation (fresh embryo transfer). However, controlled ovarian hyperstimulation induces supraphysiological hormone levels, which may adversely affect endometrial receptivity. These pathophysiological features raise concerns that the endometrial environment during fresh cycles may be suboptimal for implantation in this subgroup. Frozen embryo transfer (FET) has been proposed as an alternative approach, where embryo transfer is performed in a subsequent cycle in a more physiologic or controlled endometrial environment. For women with endometriosis, FET may theoretically reduce the adverse effects of ovarian stimulation on the endometrium and inflammatory/hormonal deregulation at the time of implantation. Extracting data of existing studies is challenged by heterogeneity in disease severity, stimulation protocols, and study design. Given the lack of consensus and the growing clinical use of FET, a systematic synthesis of the available evidence is justified. Our systematic review aims to compare reproductive outcomes following fresh versus frozen embryo transfer in women with endometriosis.