Gut and Semen Bacteria May Affect Fertility

What Did Researchers Study and Why?

Scientists at the University of Copenhagen and Copenhagen University Hospitals analysed the gut and vaginal microbiomes of women with infertility or recurrent pregnancy loss, alongside the bacteria found in their partners' semen. The goal was to understand whether these microbial communities are linked to the chances of achieving pregnancy, delivering a live baby, or experiencing pregnancy loss.

How Was the Study Conducted?

The research, published in The Lancet Obstetrics, Gynaecology & Women's Health on 4 August, followed 353 women with infertility or a history of recurrent pregnancy loss, plus 191 of their male partners. All participants were treated at specialist reproductive centres in Denmark between 2018 and 2023. According to the authors, this is one of the first studies to simultaneously examine all three microbiome sites — gut, vaginal, and semen — in couples struggling with fertility.

Why Do the Gut and Semen Matter for Fertility?

The gut microbiome influences immune responses and hormone production, both of which play a role in embryo implantation and the progression of pregnancy. The male partner's contribution is also highlighted: during intercourse, fluids and microorganisms are exchanged between partners, meaning bacteria in semen may alter the female reproductive environment — both through potential effects on sperm and through direct transmission of microorganisms.

How Big Is the Fertility Problem Globally?

The World Health Organization estimates that infertility affects at least 48 million couples worldwide. Approximately 23 million pregnancy losses occur annually, according to a 2021 study in The Lancet. Despite advances in reproductive medicine, a significant share of these cases remains unexplained — making microbiome research a promising new avenue.