Many times when a miscarriage happens, your doctor or midwife can’t tell you why. In an effort to understand what causes miscarriages, researchers have been investigating a possible link between the vaginal microbiome—the community of bacteria living in your vagina—and pregnancy loss. Here, we’ll discuss what the research says about this link.
Scientists have long known that the vaginal microbiome can affect pregnancy outcomes because it’s been shown that bacterial vaginosis—an overgrowth of bacteria in the vagina that changes the composition of the vaginal microbiome—is linked to miscarriage and to preterm birth. [1,2,3] In recent years, though, they’ve started to get a more precise idea of what specific species of bacteria typically hang out in the vagina and which of those—when they’re missing—present a potential problem for continuing pregnancies.
Understanding a typical vaginal microbiome is an ongoing process. A team of researchers led by David MacIntyre, a reproductive biologist at Imperial College London, published a study in 2015 during which they tracked the vaginal microbiomes of 42 pregnant women in the UK.  They found that that vaginal microbiome composition is dominated by a family of bacteria called Lactobacillus during pregnancy and then changes postpartum to become much less Lactobacillus heavy.
Work published in 2020 showed that a reduction in Lactobacillus in the vagina is associated with first trimester miscarriage.  And in a paper published in January 2022, a research team in Australia studied the vaginal microbiomes of 24 women during the first trimester of pregnancy.  They found that people who had been pregnant and miscarried before had a different vaginal microbial community than people who’d never been pregnant before. People who miscarried during the study also had a vaginal microbiome that was clearly different than people who carried their pregnancies to term.
In a study published in May 2022, researchers investigated which bacterial species were pregnant in the microbiomes of 63 Caucasian women who carried pregnancies to term and and 9 women who experienced first-trimester miscarriages.  They noticed less bacterial vaginosis as pregnancy continued, as well as an increase in Lactobacillus. They also found that most miscarriages were associated with an altered vaginal microbiome that looked more like bacterial vaginosis than a typical, healthy microbiome.
More support for the link between the microbiome and miscarriage came in November 2022. Researchers collected samples from the endometrium (the uterine lining) and the vagina of 47 women who’d experienced two or more consecutive pregnancy losses and 39 women without a history of pregnancy loss in Finland.  When they analyzed the composition of these microbiomes and compared the two groups of women, they found that the women with recurrent pregnancy loss had a much larger proportion of their microbiome made up of bacteria associated with bacterial vaginosis than the women without recurrent pregnancy loss.
Clearly, vaginal dysbiosis—where the vaginal microbiome is disrupted—can be a problem during pregnancy, but the question is why. New evidence from David MacIntyre and colleagues suggests that the issue is inflammation.  When the vaginal microbiome is out of whack, there is more likely to be inflammation, which can then disrupt the maintenance of pregnancy. The good news is that the microbiome is something that can be easily sampled and analyzed, meaning that it may be a modifiable target to prevent pregnancy loss in the future.
- Işik, G. et al., “Bacterial vaginosis in association with spontaneous abortion and recurrent pregnancy losses,” Journal of Cytology,
- Shimaoka, M. et al., “Association between preterm delivery and bacterial vaginosis with or without treatment,” Scientific Reports, 2019.
- Nelson, D. B. et al., “First Trimester Levels of BV-Associated Bacteria and Risk of Miscarriage Among Women Early in Pregnancy,” Maternal and Child Health Journal,
- MacIntyre, D. et al.The vaginal microbiome during pregnancy and the postpartum period in a European population. Scientific Reports.
- Al-Memar, M. et al., “The association between vaginal bacterial composition and miscarriage: a nested case-control study.” BJOG: an international journal of obstetrics and gynaecology,
- Shahid, M. et al., “Is there an association between the vaginal microbiome and first trimester miscarriage? A prospective observational study”. The journal of obstetrics and gynaecology research,
- Severgnini, M., Morselli, S., Camboni, T., Ceccarani, C., Laghi, L., Zagonari, S., Patuelli, G., Pedna, M. F. et al., “A Deep Look at the Vaginal Environment During Pregnancy and Puerperium,” Frontiers in cellular and infection microbiology, 2022.
- Peuranpää, P. et al., “Female reproductive tract microbiota and recurrent pregnancy loss: a nested case-control study,” Reproductive BioMedicine Online, 2022.
- Grewal, K. et al., “Chromosomally normal miscarriage is associated with vaginal dysbiosis and local inflammation.” BMC Medicine.