A systematic review of 40 studies found that a Lactobacillus-dominant vaginal microbiota, particularly Lactobacillus crispatus, correlates with better IVF/ICSI outcomes, while longer-term probiotic interventions showed more promise than short courses. Evidence remains heterogeneous and doesn't yet support definitive clinical recommendations for microbiota-targeted interventions.
Researchers conducted a comprehensive systematic review examining the relationship between reproductive tract microbiota composition, follicular fluid microbiota, and pregnancy outcomes in assisted reproductive technology (ART). The team screened 2,546 studies published between January 2000 and August 2025, ultimately analyzing 40 that examined microbiota profiles, microbial diversity, or probiotic interventions in women undergoing IVF or intracytoplasmic sperm injection (ICSI).
The most consistent finding across studies was straightforward: women with Lactobacillus-dominant vaginal microbiota achieved higher implantation rates, clinical pregnancy rates, and live birth rates compared to those with non-Lactobacillus or pathogen-enriched profiles. Lactobacillus crispatus appeared particularly associated with better outcomes. Conversely, microbiota compositions dominated by anaerobes or pathogenic species correlated with reduced reproductive success. This pattern held across multiple study designs, though the magnitude of effect varied considerably.
The picture becomes murkier when looking at microbial diversity alone. Studies reported inconsistent associations between overall microbial diversity and IVF outcomes, suggesting that the specific bacterial composition matters more than the number of different species present. Similarly, evidence regarding follicular fluid microbiota (the bacterial communities found in the fluid surrounding egg cells) remained limited in quantity and quality, preventing firm conclusions about its role in pregnancy outcomes.
Probiotic interventions showed differential results depending on treatment duration and approach. Longer-term or individualized probiotic regimens increased Lactobacillus abundance and were associated with improved reproductive outcomes. Short-term probiotic courses, by contrast, demonstrated minimal benefit. The heterogeneity in study designs, sampling methods, probiotic formulations, treatment durations, and outcome measures meant the authors could not perform a quantitative meta-analysis, limiting the strength of conclusions that could be drawn.
This review clarifies an emerging area of reproductive medicine while highlighting genuine knowledge gaps. For people planning IVF or ICSI, the research suggests that vaginal microbiota composition may influence success rates, but translating this into actionable guidance remains challenging.
The evidence for using probiotics specifically to improve IVF outcomes is still developing. The review found that longer courses appeared more beneficial than short-term interventions, and that interventions were more effective when individualized (likely based on baseline microbiota testing). If you're considering probiotics during fertility treatment, current evidence doesn't establish clear efficacy for improving pregnancy rates. This is an area where working with a reproductive endocrinologist familiar with microbiota research is more valuable than self-directed supplementation.
The consistent association between Lactobacillus-dominant microbiota and better outcomes is worth noting, but association is not the same as causation. We cannot yet determine whether restoring Lactobacillus dominance causes improved outcomes or whether women with naturally Lactobacillus-dominant microbiota have better outcomes for other unmeasured reasons. Large, well-designed prospective trials remain necessary to answer this question.
The limited evidence on follicular fluid microbiota deserves mention: we don't yet know whether microbiota composition within the reproductive tract influences the microbiota of follicular fluid or how either relates to embryo quality and implantation success. This represents a frontier for future research rather than actionable knowledge today.
| Category | Detail |
|---|---|
| Study Type | Systematic review and qualitative synthesis |
| Records Screened | 2,546 |
| Studies Included | 40 original human studies (RCTs and observational) |
| Population | Women undergoing IVF or ICSI |
| Time Period Covered | January 2000 to August 2025 |
| Outcomes Measured | Implantation rates, clinical pregnancy rates, live birth rates, microbiota composition, microbial diversity |
| Key Finding | Lactobacillus-dominant microbiota (especially L. crispatus) associated with higher ART success; longer-term probiotics showed more benefit than short-term courses |
| Quality Assessment | RoB 2.0, ROBINS-I, and JBI tools used; heterogeneity prevented meta-analysis |
| Journal | BMC Pregnancy and Childbirth |
| PubMed ID | 42401851 |
Study: Reproductive tract and follicular fluid microbiota and probiotic interventions in IVF/ICSI outcomes: a systematic review. BMC Pregnancy and Childbirth. PubMed: 42401851
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