Research Highlight: Lactobacillus, the Vaginal Microbiome and Miscarriage Risk

What a 2023 trial actually found, and where the headline figure applies

You may have seen a claim doing the rounds that a particular supplement "reduced miscarriage risk by 50%" for women with bacterial vaginosis (BV). It's based on a real, well-conducted study, but the way it's often summarised online mixes up two different findings within that same trial. Here's what the research actually showed.

Why Lactobacillus matters here

Lactobacillus is a beneficial bacteria that lives in both the vagina and, in smaller amounts, the endometrial cavity. It produces hydrogen peroxide, lactic acid and other compounds that keep vaginal pH below 4.5, an acidic environment that keeps less helpful organisms in check. A vaginal and endometrial environment dominated by Lactobacillus is associated in several studies with better implantation and pregnancy outcomes during fertility treatment, which is what prompted researchers to test whether adding more of it, directly, could improve outcomes.

What the study actually did

The trial (Thanaboonyawat et al., 2023, published in Scientific Reports) was a randomised controlled study run at a hospital fertility clinic in Bangkok. 340 women preparing for a frozen embryo transfer were randomised to receive either a vaginal Lactobacillus acidophilus tablet (containing 100 million CFU, alongside a very low dose of estriol) for six days before transfer, or standard care with no probiotic. 316 women were included in the final analysis.

• In the whole study group, biochemical, clinical and live birth rates were not significantly different between the two groups
• But the miscarriage rate was significantly lower in the Lactobacillus group across the whole study population: 9.5% versus 19.1%, roughly half the rate, and this is where the "reduced by 50%" figure comes from
• This finding applied to the general study population, not specifically to women with bacterial vaginosis

Where the two subgroups get mixed up

The study also ran two smaller, after-the-fact subgroup analyses, and this is where confusion creeps in. Among the 49 women specifically diagnosed with BV, live birth rate was higher in the Lactobacillus group (42.3% versus 26.1%), but this difference did not reach statistical significance, meaning it could plausibly have occurred by chance given the small sample size. Separately, among the 206 women who had a blastocyst (day 5) transfer specifically, both the miscarriage reduction (8.2% versus 24.3%) and the live birth improvement (35.7% versus 22.2%) were statistically significant. Some summaries of this study attribute the blastocyst transfer figures to the BV subgroup. That's not accurate. They're two separate groups within the same trial.

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What this means in practice

The well-supported takeaway: vaginal Lactobacillus supplementation before a frozen embryo transfer was associated with a meaningfully lower miscarriage rate in this trial, particularly for those having a blastocyst transfer, without a clear improvement in overall pregnancy or live birth rates across the whole group. The BV-specific benefit is plausible and worth further research, but this trial alone didn't prove it statistically.

The authors themselves flagged real limitations: this was a single-centre study in a Thai population, there was no placebo group, and the BV subgroup was small, which limits how confidently the findings generalise. That doesn't make the research unimportant. It adds to a growing picture that the vaginal and endometrial microbiome plays a role in pregnancy outcomes. It just supports a more specific and slightly more modest claim than "halves miscarriage risk for BV."

If you're preparing for a transfer and want to know whether this is relevant for you, test for bacterial vaginosis and broader vaginal microbiome health beforehand, rather than adding a vaginal probiotic without knowing whether there's an imbalance to correct in the first place.

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