Bacteria in Semen May Raise Risk of Poor Pregnancy Outcomes: Study

Infertility has historically been viewed largely as a female health issue, despite conception depending on both partners. New research is highlighting another potential contributor to fertility: the microbiome of semen.

A recent Danish study published in The Lancet Obstetrics, Gynaecology, and Women’s Health investigated the gut, vaginal and semen microbiomes of couples undergoing fertility treatment and couples being evaluated for recurrent pregnancy loss.

Researchers analyzed samples from 353 women and 191 male partners using deep shotgun metagenomic sequencing to identify the microorganisms present in the different body sites.

The study found that an imbalance in semen bacteria, known as seminal V-dysbiosis, was present in about 24% of men in the IVF group and 41% of men in the recurrent miscarriage group.

Among couples undergoing IVF, outcomes also differed depending on the male partner’s semen microbiome. Around 85% of couples achieved a live birth when the male partner had a balanced semen microbiome, compared with 57% when seminal dysbiosis was present.

The semen microbiome refers to the community of microorganisms living in seminal fluid. It includes bacteria such as Lactobacillus, Prevotella and Pseudomonas. Once considered sterile, semen is now known to contain a diverse microbial community originating from areas including the reproductive and urinary tracts.

Certain microbial patterns may be associated with sperm health. Higher levels of Lactobacillus have been linked with better sperm movement and concentration, while an overgrowth of certain bacteria, including Prevotella, Ureaplasma and some Pseudomonas strains, has been associated with poorer sperm quality and motility.

The findings suggest that male reproductive health and the semen microbiome could be important factors in fertility treatment outcomes. However, the study demonstrates an association and does not prove that correcting seminal dysbiosis will improve IVF success. Further research is needed to determine whether microbiome-based testing or treatments could eventually become part of fertility care.

REFERENCE: Behrendt-Møller I, Krog M, Myers P et al.; Gut, vaginal, and seminal microbiomes in couples with infertility or recurrent pregnancy loss: a two-cohort prospective study of reproductive outcomes; The Lancet Obstetrics, Gynaecology, & Women’s Health, 2026; DOI: 10.1016/S3050-5038(26)00213-X