The use of Lactobacillus-containing probiotics may help improve pregnancy rates among women undergoing assisted reproductive technology (ART), according to a systematic review and meta-analysis.
Pooled data from seven randomized controlled trials involving infertile women undergoing ART (n=796) and published between 2023 and 2025 showed that the rate of clinical pregnancy was 41.98 among those who received Lactobacillus-containing probiotics (n=399) vs 28.25 percent among those who received placebo or no treatment (n=397). [ESHRE 2026, abstract L26/P-473]
The likelihood of achieving a clinical pregnancy after ART among women who received Lactobacillus-containing probiotics was 49 percent higher than that observed among women who received placebo or no treatment (risk ratio [RR], 1.49, 95 percent confidence interval [CI], 1.07–2.07; I²=29 percent).
The absolute effect associated with Lactobacillus-containing probiotics was 12.4 clinical pregnancies per 100 women treated, with an approximate number needed to treat of 8.1.
Study authors Dr Daiki Hiratsuka and Dr Yasushi Hirota from the University of Tokyo, Tokyo, Japan, noted that the positive association between Lactobacillus-containing probiotics and clinical pregnancy was stronger when the probiotics were used for more than 2 weeks (RR, 1.99, 95 percent CI, 1.44–2.75; I²=0 percent) than when used for up to 2 weeks only (RR, 1.16, 95 percent CI, 0.67–2.00).
In an exploratory synthesis of three observational 16S rRNA studies (n=67), Hiratsuka and Hirota found that the use of the probiotics converted non-Lactobacillus-dominant endometrial microbiota to a Lactobacillus-dominant status in 79 percent of infertile women. However, the risk of bias for this finding was either moderate or high, they added.
“No adverse events were reported in the included trials, although adverse-event reporting was limited,” the authors said.
Taken together, the present data highlight the potential of Lactobacillus-containing probiotics as a noninvasive adjunct to improve ART outcomes, particularly when treatment is given beyond 2 weeks and in women with non-Lactobacillus-dominant endometrial microbiota, according to Hiratsuka and Hirota.
The authors acknowledged several limitations to the meta-analysis. Eligible trials were few, live birth was rarely reported, and definitions of clinical pregnancy might have varied across studies. Additionally, several trials had blinding/reporting limitations, and the microbiota endpoint relied on observational designs vulnerable to confounding and selection bias.
“[Our findings] provide a rationale for larger, multicentre, well-blinded trials with standardized probiotic regimens, baseline microbiota stratification, and robust assessment of live birth and safety,” Hiratsuka and Hirota said.
“Clarifying which patients may benefit most—particularly those with non-Lactobacillus-dominant profiles—will be essential to support targeted, evidence-based integration of probiotics into ART practice,” they added.
Endometrial Lactobacillus abundance
A Lactobacillus-dominated microbiota profile in the female reproductive tract has already been shown to play an important role in reproductive health, having been associated with higher rates of both natural conception and improved outcomes in ART. [Int J Mol Sci 2024;25:622; Reproduction 2022;163:R81-R96]
Recently, a group of researchers from China reported that endometrial Lactobacillus abundance decreased with advancing age, particularly past age 35 years. Analysis of endometrial samples from 149 women ages 20 to 45 years who were being treated for fertility showed that those in the <30-year age group had a significantly higher abundance of Lactobacillus bacteria than those in the 30–35-year and >35-year age groups. [Cell Host Microbe 2026;doi:10.1016/j.chom.2026.06.018]
Over 12 months of follow-up, 93 women received embryo transfers, which resulted in 56 clinical pregnancies. Clinical pregnancy rates did not significantly differ across the <30-year, the 30–35-year, and >35-year age groups. However, women who achieved a clinical pregnancy tended to have higher endometrial Lactobacillus abundance than those who had unsuccessful embryo transfers.
“Together, these observations indicate that a beneficial Lactobacillus-enriched endometrial microbiome may be associated with more favourable implantation outcomes. However, these findings are based on observational data from a single-centre Chinese cohort and may have limited generalizability,” the researchers said.