Implantation failure: is the gut microbiome the missing suspect?
Recurrent implantation failure workups focus on the embryo, the endometrium and, lately, the vaginal microbiota. A pilot study now looks at the gut and finds a less diverse ecosystem, poorer in protective, SCFA-producing bacteria.
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Infertility affects about 17.5% of adults worldwide, and assisted reproduction still fails more often than it succeeds: fewer than half of IVF cycles end in a live birth. Even euploid blastocysts fail to implant in 40 to 50% of cases. Implantation remains the “black box” of IVF, and when failures repeat, the search usually turns to the embryo, the endometrium or the vaginal microbiota. A pilot study¹ from Tor Vergata University of Rome, published in Microorganisms, looked further from the uterus: at the gut.
Short-chain fatty acids in brief
When dietary fiber reaches the colon, bacteria such as Faecalibacterium, Roseburia and Anaerostipes ferment it into butyrate, propionate and acetate. Butyrate is the main fuel of colonocytes, it reinforces tight junctions and it promotes regulatory T cells. Several of these producers were depleted in the women with recurrent implantation failure in this study.
A narrower, differently organized ecosystem
The team1 compared 20 women with recurrent implantation failure (RIF), defined according to the 2023 ESHRE recommendations as repeated unsuccessful embryo transfers warranting further investigation, with 21 fertile controls of similar age (mean 38.5 years). Stool was collected within a year of the last failed transfer and profiled by 16S rRNA sequencing. Of 141 operational taxonomic units detected, 50 appeared only in controls and 32 only in women with RIF. Women with RIF also had lower alpha diversity (Shannon p = 0.003; inverse Simpson p = 0.002), and their communities clustered apart from those of controls (PERMANOVA R² = 0.16; p = 0.001). The separation held after adjustment for clinical covariates such as waist-to-hip ratio, though group then explained about 5% of the variance.
Under the 2023 ESHRE good practice recommendations, RIF describes a history of at least two embryo transfers that did not lead to a positive pregnancy test, at a frequency high enough to warrant investigation. The women in this study met that definition, with an average of 2.5 blastocysts that had failed to implant.
Alpha diversity tells us how many taxa live in one person's gut and how evenly they share the space; beta diversity compares whole communities between people. Here both pointed the same way: women with RIF had less diverse gut microbial communities, with a different overall composition compared with fertile controls.
Butyrate, propionate and acetate are produced when gut bacteria ferment dietary fiber. They feed the colonic epithelium, help keep the intestinal barrier tight and support immune tolerance, which is why the loss of SCFA-producing bacteria in women with RIF caught the authors' attention.
Fewer fiber fermenters, more inflammatory signals
Firmicutes fell from 65.0% to 52.7% of the community. Within that phylum, barrier-supporting genera such as Faecalibacterium and Roseburia made up a smaller share, the fiber-fermenting Lachnospiraceae and Peptostreptococcaceae families were depleted (the latter including Romboutsia, a genus involved in bile acid metabolism), and butyrate producers such as Anaerobutyricum, Anaerostipes and Fusicatenibacter were significantly reduced. Not every shift fits a simple good-versus-bad reading: Bacteroides, usually seen as a healthy commensal that produces acetate and propionate, was more abundant in the RIF group. In excess, it can degrade the mucus layer and add lipopolysaccharide to the gut. Proteobacteria, often associated with gut dysbiosis, rose from 3.6% to 9.1% (p < 0.001), with Enterobacteriaceae, Desulfovibrionaceae and the hydrogen sulfide producer Bilophila enriched. The authors suggest this microbial signature could sustain chronic low-grade inflammation through lipopolysaccharide signaling via TLR4, weaken the intestinal barrier and perhaps disturb estrogen recycling by the estrobolome. These pathways remain hypotheses: the study measured bacteria, not metabolites or immune markers.
A phylum of Gram-negative bacteria, including Escherichia and Klebsiella, whose outer membrane carries lipopolysaccharide, a potent trigger of inflammation. An expansion of this group is a common sign of dysbiosis; in this study it was about 2.5 times more abundant in women with RIF.
The epithelium, its tight junctions and the mucus layer that keep gut microbes away from the bloodstream. Hydrogen sulfide released by bacteria such as Bilophila can erode the mucus layer, one of the routes through which the authors think gut dysbiosis might feed chronic inflammation.
The collection of gut bacterial genes, notably those encoding β-glucuronidase, that reactivate estrogens excreted in bile and send them back into circulation. The study did not measure it, but it offers a plausible bridge between the gut microbiota and hormonal balance in women trying to conceive.
Marker, target or bystander?
The design limits what the data can say. Controls were naturally fertile women, not IVF patients with a successful transfer, so the signature may reflect infertility, ART exposure or reproductive history rather than failed embryo implantation itself. Pharmacological data such as metformin use were missing, and 16S profiling says nothing about function. Still, the gut has one practical advantage over the uterus: it can be sampled non-invasively and potentially modulated through diet, probiotics or lifestyle interventions, making it an interesting target for future research. For now, the gut microbiota belongs on the RIF research agenda, not yet on the prescription pad.
1. La Placa G, Fabozzi G, Pala B, Peluso D, Cimadomo D, Vaiarelli A, et al. Exploring the association between gut microbiota and infertility in women with multiple implantation failures: an exploratory study. Microorganisms. 2026;14(6):1334. doi:10.3390/microorganisms14061334.
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