P-465 Patients with Repeated Implantation Failure do not have a higher prevalence of an unfavourable vaginal microbiome

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Abstract Study question What is the prevalence of an unfavourable vaginal microbiome among women experiencing Recurrent Implantation Failure (RIF) compared with controls? Summary answer The prevalence of an unfavourable vaginal microbiome among women experiencing RIF was significantly lower than the prevalence in the control group (21% versus 53%, p = 0.001). What is known already Infertility affects one in six couples and is recognised as a global health issue by the World Health Organisation. Although Medical Assisted Reproduction (MAR) offers successful treatment for most couples, 4-10% will experience Repeated Implantation Failure, a condition termed RIF. Increasing research sheds light on this clinical phenomenon, however a widely accepted definition is lacking. Furthermore, a multifactorial pathogenesis is considered, including disruption of the vaginal microbiome with a relatively low load of Lactobacillus and a high load of Proteobacteria, Gardnerella Vaginalis or Lactobacillus Jensenii leading to decreased pregnancy chances. This stratification is reflected in the receptIVFity test. Study design, size, duration RIF-patients and control subjects were recruited in a single-centre University observational cohort study between 2019 and 2024. RIF was defined as consecutive implantation failure of three high quality embryos, based on the Gardner grading scale, or ten embryos without quality criterion. Women referred because of a severe male factor, bilateral tubal pathology or preimplantation genetic testing (PGT) for hereditary disorders without concurrent subfertility were recruited as a control group. Participants/materials, setting, methods Clinical characteristics and vaginal swabs were collected 5-8 days after the LH-peak in natural cycles. The vaginal microbiome was analysed using the IS-Pro based ReceptIVFity test (ARTPred, Amsterdam, the Netherlands). For the composition of the vaginal microbiome a previously described algorithm characterized the microbiome as either high, medium, or low implantation potential. Means were compared by the independent t-test and categorical variables by the chi-square test with SPSS 28.0. P-value <0.05 was considered statistically significant. Main results and the role of chance The clinical characteristics showed that women with RIF (n = 81) were older (33.5 years vs 30.2 years, p < 0.001) in comparison with the control subjects (n = 32). No significant differences were found in BMI, ethnicity and smoking habits. In the RIF-patients, the most common cause of infertility was male factor (44%), followed by female factor (28%), and an unknown factor (28%). In the control subjects the highest reason for referral to our centre was PGT (78%), followed by severe male factor (16%) and tubal pathology (6%). When compared to the control subjects, RIF-patients had a lower prevalence of an unfavourable vaginal microbiome (low profile, 21% vs 53% respectively, p = 0.001). However, the composition differs. In subjects with an unfavourable vaginal microbiome, the relative abundance of Lactobacillus Crispatus is lower in women experiencing RIF compared to control subjects (18% vs 54%, p = 0.119). Moreover, the relative abundance of Proteobacteria is higher among RIF-patients with an unfavourable vaginal microbiome compared to control subjects (14% vs 4%, p = 0.040). Limitations, reasons for caution There is no widely accepted definition of RIF which makes the group heterogeneous. Furthermore, since genetic testing for aneuploidy was not performed, it cannot be ruled out that an embryo factor instead of an endometrial factor caused RIF. Wider implications of the findings The findings show that the vaginal microbiome is more often unfavourable for implantation in control subjects compared to RIF-patients. Thus the question remains if an aberrant microbiome contributes to RIF. Trial registration number Yes

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