Endometriosis is an estrogen-dependent, chronic, and often painful gynecological disorder characterized by the presence of endometrial-like tissue outside the uterine cavity [1]. It affects 10–15% of females of reproductive age and is known to have a detrimental impact on fertility [2, 3].

The underlying causes of infertility in endometriosis remain elusive, despite various proposed mechanisms [4]. One plausible way in which endometriosis impairs fertility and compromises the chances of achieving a successful pregnancy is by adversely affecting the quality and quantity of oocytes and embryos [5]. While in vitro fertilization (IVF) is a recognized and effective treatment option for endometriosis-related infertility [6], several studies have shown that individuals experiencing endometriosis-associated infertility undergoing IVF treatments exhibit a significant decline in the total number of oocytes, the number of mature oocytes, and the levels of nuclear and cytoplasmic oocyte maturation. Additionally, they experience reduced rates of fertilization, implantation, and clinical pregnancy [7, 8, 9, 10].

Some studies suggest that the severity of endometriosis can significantly affect the success of IVF treatments [11, 12]. A notable decrease in fertilization rates has been observed in the advanced stages (stages III and IV) of endometriosis compared to the milder stages (stages I and II) [13]. Pellicer and colleagues found that women with stage III–IV endometriosis had fewer blastomeres per embryo and a higher frequency of embryos arrested in development [12]. However, other studies have failed to identify any significant effects on indicators of reproductive success [14, 15].

Notably, peritoneal fluid from women with endometriosis has been found to inhibit the binding of spermatozoa to the zona pellucida [16]. In light of these challenges, it has been suggested that intracytoplasmic sperm injection (ICSI), originally developed for severe male factor infertility, could be applied to non-male factor infertile patients, including those with previous complete fertilization failure or low fertilization rates in conventional IVF cycles. Split insemination of oocytes using both IVF and ICSI has been recommended for patients with unexplained infertility undergoing IVF treatment, as a strategy to mitigate the risk of fertilization failure after IVF [17].

Given the lack of clear evidence on the optimal fertilization method for patients with different stages of endometriosis, this study compared the outcomes of conventional IVF after short in vitro maturation (IVM) and ICSI, focusing on fertilization rates, embryo quality, and development to provide more insightful comparisons.

In the current study, we examined patients with different stages of endometriosis and partners with normozoospermic semen, in whom sibling oocytes were randomly inseminated either by conventional IVF following short IVM or ICSI. The findings revealed the following key observations: (i) the dropout rate and complete fertilization failure were notably higher in the stage III–IV endometriosis group compared to the stage I–II endometriosis group; (ii) within the stage I–II endometriosis group, basal serum AMH levels, the mean number of retrieved oocytes, as well as the rates of maturity and fertilization were significantly higher than in the stage III–IV endometriosis group; (iii) despite the insemination method, there were no significant differences in fertilization rates, embryo formation, or the proportion of day 3 top-quality embryos within each endometriosis group.

Studies have indicated that the severity of endometriosis, categorized into stages I to IV, may influence the quality of follicles/oocytes, embryo development, and their viability [2, 19]. In cases of minimal to mild (stage I–II) endometriosis, evidence suggests that oocyte and embryo quality may be relatively preserved, with fewer impairments in fertilization and embryo development. However, in more advanced stages (stage III–IV) of endometriosis, there appears to be an increased risk of reduced ovarian reserve and oocyte numbers, a higher likelihood of cycle cancellations, and compromised oocyte and embryo quality [2, 20]. This may be attributed to increased pelvic inflammation, higher oxidative stress, disruptions in steroid synthesis, and altered ovarian function [21, 22]. These factors collectively affect the follicular environment, leading to an increased risk of meiotic abnormalities, chromosomal instability, and alterations in oocyte morphology [23]. Consequently, these changes compromise oocyte quality, which, in turn, can impair embryo development.

Consistent with some previous observations, our findings also showed that as endometriosis progresses, there is a decrease in the number of retrieved oocytes, mature oocytes, and fertilization rates, along with an increase in complete fertilization failure. However, we did not find a significant difference in the rates of embryo formation and top-quality embryos. Dong et al. [24] revealed that, despite a significantly lower number of retrieved oocytes, there were no statistically significant differences between the stage III–IV endometriosis group and the stage I–II group regarding fertilization rate and the rate of high-quality embryos on day 3. A recent systematic review and meta-analysis of 22 studies assessing the influence of endometriosis on embryo quality also found that in women with stage III–IV endometriosis, there were no statistically significant differences in high-quality embryo rates, cleavage rates, or embryo formation rates when compared to those without endometriosis [25]. Women who undergo surgery for moderate to severe endometriosis have been reported to have a comparable number of cleavage embryos and good-quality embryos when compared to women without endometriosis [26]. Conversely, Pellicer and colleagues [12] discovered that women with stage III–IV endometriosis exhibited reduced embryo quality and a higher incidence of arrested embryos. Nevertheless, our findings on embryo quality should be interpreted with caution, as there are currently no universally accepted criteria or terminology for embryo morphological assessment worldwide. Moreover, the morphological grading of embryos is highly subjective, which limits its predictive certainty [27].

Notably, as the development of the human embryo is directly influenced by both the nuclear and cytoplasmic maturation of the oocyte, and considering that endometriosis can impair the quality of the surrounding follicular fluid and, subsequently, the oocytes maturation environment, we implemented a brief IVM period after oocyte retrieval and before commencing IVF to enhance oocyte maturation. Interestingly, we observed similar fertilization rates, embryo formation, and production of high-quality day 3 embryos when comparing sibling oocytes subjected to ICSI or IVF following the short IVM period, in both stage I–II and stage III–IV groups. In contrast, a previous study by Komsky-Elbaz et al. [1] found a significantly higher fertilization rate of oocytes inseminated by ICSI, yielding a significantly higher mean number of day 2 embryos, as well as a higher mean number of embryos suitable for cryopreservation, compared to conventional IVF in patients with severe endometriosis and normozoospermic semen. For most IVF programs, higher rates of fertilization and embryo formation are expected following ICSI compared to conventional IVF. However, due to the lack of detailed information regarding culture conditions in the previous study, this discrepancy in our findings may be cautiously attributed to the short IVM period employed in our approach.

The present study has several limitations that should be noted. Firstly, we did not assess the rate of blastocyst formation, which serves as a critical indicator of embryo developmental competence. Additionally, our study did not consider implantation rates, clinical pregnancy rates, or live birth rates as primary outcome measures due to the inclusion of patients undergoing a frozen embryo transfer strategy. A strength of the present study was the use of the sibling oocyte model to evaluate the most effective ART approach in endometriosis, as interpatient variability is a significant confounding factor.

Our findings suggest that conventional IVF following a short IVM period produces comparable outcomes to ICSI in patients with varying stages of endometriosis. This indicates that IVF with a brief IVM period can serve as an equally effective, yet less invasive, alternative to ICSI for treating endometriosis-associated infertility.

The data that support the findings of this study are not publicly available due to privacy reason but are available from the corresponding author upon reasonable request.

MM, HGT, EN, and MarzD designed the research study. MarzD and MaryD performed the research. SGS did the data collection. MarzD and ES analyzed the data. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

The study was approved by the Ethics Committee, Isfahan University of Medical Sciences, Isfahan, Iran (Approval ID: IR.MUI.MED.REC.1402.313) and confidentiality of the data was ensured. This study was conducted in accordance with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. Informed consent was obtained from all participants included in the study.

This work was supported by Isfahan University of Medical Sciences (Grant number: 3402437).

The authors declare no conflict of interest.

During the preparation of this work the authors used ChatGpt-3.5 in order to check spell and grammar. After using this tool, the authors reviewed and edited the content as needed and takes full responsibility for the content of the publication.