Exploring the Association Between Gut Microbiota and Infertility in Women with Multiple Implantation Failures: An Exploratory Study

Implantation failure remains a major challenge in IVF, and the contribution of the gut microbiota to implantation success is still poorly defined. We conducted a pilot matched case-control study (February 2023-December 2024) to compare gut microbiota profiles between women with RIF (defined according to ESHRE good practice recommendations) and fertile controls with documented fertility (≥1 prior spontaneous pregnancy). All participants underwent standardized clinical and nutritional assessment of medical history, dietary habits, anthropometry, and body composition. Stool samples were collected for 16S rRNA gene sequencing. In women with RIF, sampling occurred within 1 year after the last failed embryo transfer. Of 45 enrolled women, 41 completed the study (20 RIF and 21 controls; mean age 38.46 ± 4.53 years), with no significant between-group age differences. Women with RIF showed reduced alpha diversity (Shannon p = 0.003; inverse Simpson p = 0.002) and a distinct community structure versus controls (Bray-Curtis PERMANOVA F = 7.16; R2 = 0.16; p = 0.001), which remained significant after adjustment for clinical covariates including waist-to-hip ratio (p = 0.018). At the phylum level, women with RIF had fewer Firmicutes (52.7% vs. 65.0%; p = 0.012) and more Proteobacteria (9.1% vs. 3.6%; p < 0.001). These findings support an association between gut dysbiosis and a history of implantation failures and warrant confirmation in larger, longitudinal cohorts.