News release

From: The Lancet

The Lancet Obstetrics, Gynaecology, & Women’s Health: Bacterial imbalance in semen linked to miscarriages and fewer births, study suggests

An imbalance in the bacteria found in men’s semen was linked to substantially lower chances of a couple having a baby, and to more pregnancy losses, according to new research published in The Lancet Obstetrics, Gynaecology, & Women’s Health journal. This association between semen bacteria and birth outcomes was seen in couples undergoing in vitro fertilisation (IVF) or intracytoplasmic sperm injection (ICSI) fertility treatment as well as women with recurrent pregnancy loss. An imbalance in women’s gut bacteria [1] was linked to a longer time to conceive and a lower likelihood of conception.

The study followed 353 women and 191 of their male partners [2], all heterosexual couples recruited between April 2018 and September 2023, who were undergoing fertility treatment or had experienced recurrent pregnancy losses. Researchers analysed the bacteria in the women’s gut and vagina and in the men’s semen, then tracked each couple’s pregnancies and births over time.

About a quarter of the male partners had a bacterial imbalance in their semen. Among couples undergoing IVF/ICSI, 57% of couples where the man had this imbalance went on to have a baby, compared with 85% where it was not detected. Pregnancy loss was also more common when the man had the imbalance, affecting roughly one in five more couples than when it was not detected. Importantly, a standard semen analysis cannot detect this imbalance, so a man’s results can look normal even when it is present.

In women, an imbalance in the gut bacteria was linked to a lower chance of becoming pregnant through fertility treatment, and to taking longer to conceive. It was not, however, linked to miscarriage. An imbalance in the vaginal bacteria was not clearly linked to either infertility or pregnancy loss.

The authors say the findings point to imbalances in men’s semen and in women’s gut bacteria as previously overlooked and potentially treatable risk factors for infertility and recurrent pregnancy loss.

[1] These communities of bacteria are known as the microbiome; an unhealthy imbalance in them is called dysbiosis.

[2] Fewer men than women were included in the analysis because a male partner contributed only when a semen sample was available. For example, some couples used donor sperm whereas every woman provided samples and pregnancy outcomes.