ART and embryo transfer: are lactobacilli the key to success?

In assisted reproductive technology (ART), embryo implantation failures remain difficult to explain, probably because they involve multiple factors. Among these, a vaginal microbiota dominated by Lactobacillus seems to enhance the success of embryo transfers. However, results can sometimes be contradictory, and the ethnic origin of women is rarely considered.

A prospective single-site observational study¹ (Phoenix, USA) has therefore analyzed the vaginal microbiota of 87 American women during frozen embryo transfers, including 15 women of Hispanic origin. Its goal: to better understand the impact of vaginal flora on pregnancy rates after frozen embryo transfer, highlighting the protective role of Lactobacillus, while considering ethnic diversity.

More Lactobacillus, more pregnancies

Of the 55 patients who became pregnant, two-thirds (67%, or 37 women) exhibited a microbiota dominated by Lactobacillus at the time of insemination. These women had a 52% higher chance of becoming pregnant compared with those whose flora was not dominated by lactobacilli. Patients who did not become pregnant had more opportunistic pathogens, notably species of Enterobacteriaceae and Streptococcus.

In contrast, the richness or diversity of the vaginal flora was unrelated to ART outcome. Thus, the vaginal microbiota appears to interact with female fertility and the outcome of frozen embryo transfer: vaginal microbiomes dominated by Lactobacillus, especially those where the species L. crispatus or L. gasseri are prevalent, are positively associated with pregnancy. 

Explaining ethnic disparities?

The study also examined ethnic disparities. Hispanic women, who made up 18.3% of the cohort’s women (and about 19% of the US population), had lower clinical pregnancy rates following embryo transfer, a trend also observed nationally.

Moreover, a smaller proportion of them had a vaginal microbiota dominated by lactobacilli (compared with non-Hispanic white women), an observation already reported in previous studies. Could this lower prevalence of Lactobacillus dominance among Hispanic women partly explain the lower success of embryo transfer in this population? That is indeed the researchers’ hypothesis.