Why Bacterial Vaginosis (BV) can work against conception and a healthy pregnancy.

Trying to conceive and wondering about your vaginal microbiome? You're asking the right question.

A growing body of research shows that the balance of bacteria in the vagina plays a role in fertility and pregnancy outcomes. And Bacterial Vaginosis (BV) is a particularly common form of 'imbalance' women can experience, either with symptoms or without (50 - 75% of cases of BV are asymptomatic).

The research: Lactobacillus dominance and better pregnancy outcomes.

A healthy vaginal microbiome is typically dominated by Lactobacillus species, acid producing bacteria that keep the vaginal environment healthy and free from opportunistic microbes. The research linking this dominance to positive reproductive outcomes has grown substantially in recent years:

• A 2025 longitudinal study of a Japanese maternal-neonatal cohort found that higher Lactobacillus abundance in early pregnancy was associated with a more favourable gestational period, with L. crispatus the consistently dominant species across pregnancy (Nature Communications, 2025).
• In IVF and fertility treatment settings, a Lactobacillus crispatus-dominant cervical microbiome has been linked to significantly higher biochemical and clinical pregnancy rates compared with environments dominated by other bacteria (Frontiers in Cellular and Infection Microbiology, 2023).
• A cervical microbiome study of women undergoing frozen embryo transfer found that those with an L. crispatus-dominant profile had a meaningfully higher pregnancy rate than those with L. iners-dominant or mixed-bacteria profiles.

Overwhelmingly, the research points to a stable, Lactobacillus-rich vaginal environment being associated with better reproductive outcomes.

What is BV and how common is it?

BV happens when protective Lactobacillus species are replaced by an overgrowth of anaerobic bacteria, such as Gardnerella, Sneathia and Prevotella species. 

BV is common in reproductive aged women, estimated in about 23% of women in European countries.

When symptomatic, BV can cause an unusual grey or white discharge, an off or fishy odour (often more noticeable after sex), and mild irritation.

But here's the annoying bit. You can have the underlying bacterial imbalance with no symptoms.  This matters enormously in a pregnancy context, because an undetected imbalance can still be influencing a woman's reproductive environment even when everything feels normal.

The mechanics: why BV works against a supportive pregnancy environment.

There are several biological mechanisms by which BV can work against conception and a healthy pregnancy:

1) Altered discharge and cervical mucus may affect sperm function.

Research modelling the cervix in the lab has shown that a BV-associated bacterial environment reduces the thickness of the protective cervical mucus layer and is linked to reduced sperm motility. Toxins produced by BV-associated bacteria have also been shown to impair sperm capacitation (the process sperm need to undergo before they're able to fertilise an egg).

2) Local inflammation and immune dysfunction may hinder implantation.

BV is associated with raised levels of pro-inflammatory cytokines (IL-1β, IL-6, IL-8) in the reproductive tract. This chronic low-grade inflammation, along with direct effects of bacterial toxins on the endometrium, has been linked to impaired embryo implantation and early pregnancy loss.

There are also mechanisms that come into play once bacteria are established, which matter further along the pregnancy timeline:

3) Enzymes that break down the mucus plug.

Gardnerella and Prevotella (BV microbes) produce sialidase and mucinase enzymes that actively degrade the cervical mucus and weaken the mucosal barrier. Elevated sialidase activity specifically has been linked to a higher risk of preterm birth.

4) Ascending infection into the uterus.

Once the cervical mucus barrier is weakened, bacteria don't necessarily stay in the vagina, they can travel upward into the uterine cavity itself. This ascending route is thought to be involved in a substantial proportion of preterm births associated with infection.

BV during pregnancy has been linked to more serious downstream risks:

Preterm birth

Meta-analyses have consistently found that BV roughly doubles the risk of preterm delivery, with earlier detection (before 16–20 weeks) associated with even higher risk in some studies. Mechanistically, this is thought to run through inflammation triggering matrix metalloproteinases (MMPs) that break down collagen in the fetal membranes and cervix, weakening the membranes (a known contributor to premature rupture of membranes) and encouraging the cervix to shorten and ripen earlier than it should.

Miscarriage

Earlier research found BV was associated with a substantially increased risk of spontaneous abortion when present in the first trimester, and more recent reviews continue to support a meaningful association between BV and pregnancy loss.

Impact on the newborn's microbiome

Babies born vaginally acquire much of their first bacterial exposure from their mother's vaginal microbiome, and this early exposure is thought to help shape the infant's developing gut and immune system. When that maternal environment is BV-dominant rather than Lactobacillus-dominant, the bacteria available for this first "seeding" are simply less favourable.

The vaginal microbiome is just one part of the puzzle when it comes to pregnancy outcomes; the good news being it can be optimised relatively easily, so is certainly an area to consider in those planning a pregnancy and those having difficulty conceiving.

What women can do if they are worried about BV.

Test, don't assume.

Because more than half of BV cases have no symptoms, "I feel fine" isn't a reliable way to rule it out. A vaginal microbiome test can identify an imbalance before it becomes symptomatic or has a chance to affect your reproductive environment.

Consider a probiotic to support healthy Lactobacillus levels.

Restoring and maintaining Lactobacillus dominance is an active area of clinical research. A landmark randomised, placebo-controlled trial found that a Lactobacillus crispatus vaginal probiotic significantly reduced BV recurrence after antibiotic treatment.

A broader systematic review and meta-analysis of randomised trials has likewise found that probiotics containing Lactobacillus strains help reduce BV recurrence rates. This matters particularly because recurrence after antibiotic treatment alone is common, sometimes affecting more than half of women within a year.

We recommend Ultimate V, containing lactobacilli species native to a healthy vaginal canal, as a way to promote health in the vaginal canal.

Summary

Bacterial vaginosis is common, often silent, and worth taking seriously if you're planning a pregnancy or trying to conceive. But it's also very manageable once you know it's there. Knowledge is power, and we hope this blog has helped you think a little more broadly about reproductive health. The right tests and tools exist to help you understand what's going on and take steps to support it, so you don't have to leave it to guesswork.

If you have any questions please don't hesitate to contact us: hello@pelvicrelief.co.uk.

This article is intended for general education and does not replace individual medical advice. If you have symptoms of BV or are concerned about your vaginal health during pregnancy or while trying to conceive, please speak with your GP or a specialist.

References 

• Oguri N, Kobayashi C, Ozawa Y, et al (2025). Vaginal Lactobacillus crispatus in early pregnancy associates with favorable gestational outcomes in a Japanese maternal-neonatal microbiome cohort. Nature Communications. doi: 10.1038/s41467-025-63466-3
• Guan W, Dong S, Wang Z, Jiao J & Wang X (2023). Impact of a Lactobacillus dominant cervical microbiome, based on 16S-FAST profiling, on the reproductive outcomes of IVF patients. Frontiers in Cellular and Infection Microbiology. doi: 10.3389/fcimb.2023.1059339
• Carlson K, Mikes BA & Garg M (2025). Bacterial Vaginosis. StatPearls [Internet]. NBK459216
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