Bacterial vaginosis (BV) develops when the balance of bacteria inside the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment and keep it slightly acidic. When those bacteria decline sharply, anaerobic bacteria (the kind that thrive without oxygen) multiply and take over. This imbalance is what produces the symptoms of BV, and it’s the most common vaginal condition in women ages 15 to 44.
What triggers that shift isn’t always obvious, and for many women, there’s no single clear cause. But research has identified several factors that reliably increase the risk.
What Happens Inside the Vagina
A healthy vagina is home to large colonies of lactobacilli, which produce hydrogen peroxide and lactic acid. These create a mildly acidic environment (a pH below 4.5) that keeps other bacteria in check. When something disrupts that ecosystem, lactobacilli numbers drop and anaerobic bacteria fill the gap. The vaginal pH rises above 4.5, and a thin, grayish-white discharge with a fishy odor often follows.
The most common bacterium found in BV is Gardnerella vaginalis. It forms a sticky biofilm along the vaginal walls that shelters itself and other BV-associated microbes from the body’s normal defenses. This biofilm is one reason BV so frequently comes back after treatment: even when antibiotics clear the symptoms, the biofilm can persist and allow the infection to re-establish itself.
Sexual Activity Is a Major Factor
For years, BV was not classified as a sexually transmitted infection because it also occurs in women who have never had sex. That picture has started to change. Researchers at Indiana University School of Medicine confirmed in 2023 that the bacteria responsible for BV can be sexually transmitted through vaginal intercourse. Their study found that only men who reported having vaginal sex carried BV-associated bacteria, while men who had not did not carry them. This was the first research to directly demonstrate sexual transmission of BV.
New or multiple sexual partners have long been recognized as one of the strongest risk factors. Each new partner introduces a different microbial community, which can destabilize the existing vaginal flora. Women who have sex with women also experience higher rates of BV, likely because the bacteria transfer between partners during intimate contact. That said, BV still isn’t categorized the same way as chlamydia or gonorrhea, because it can develop without any sexual contact at all. The current understanding is that sex is a common trigger, but not the only one.
Douching Disrupts the Vaginal Environment
Douching is one of the most well-documented non-sexual risk factors. Rinsing the inside of the vagina with water, vinegar, or commercial douching products directly disturbs the protective systems that rely on hydrogen peroxide and lactobacilli. Once that defense is stripped away, anaerobic and aerobic bacteria overgrow and BV can develop.
The same principle applies to scented soaps, body washes, and feminine hygiene sprays used inside or around the vagina. These products alter the vaginal pH and chemical environment. The vagina is self-cleaning, and introducing outside products does more harm than good. Washing the external vulva with plain water or a mild, unscented soap is sufficient.
Smoking and Other Risk Factors
Cigarette smoking increases BV risk, though the exact mechanism isn’t fully understood. One theory is that chemicals absorbed from tobacco reduce the concentration of lactobacilli in the vaginal lining, making the environment more hospitable to anaerobic bacteria. Women who smoke are consistently more likely to develop BV and to have recurrent episodes.
Other factors that can tip the balance include:
- Recent antibiotic use: Antibiotics taken for unrelated infections (a sinus infection, urinary tract infection, etc.) can kill off vaginal lactobacilli along with their intended targets, opening the door for BV-associated bacteria.
- IUD use: Some studies have found a modest association between intrauterine devices and BV, possibly because the string provides a surface for biofilm formation.
- Hormonal changes: Fluctuations in estrogen during menstruation, pregnancy, or menopause affect the vaginal environment and can influence which bacteria thrive.
How BV Is Recognized
Many women with BV have no symptoms at all. When symptoms do appear, the most common is a thin, milky discharge that coats the vaginal walls, often accompanied by a fishy smell that tends to be stronger after sex. There’s usually no itching or burning, which helps distinguish BV from a yeast infection.
Clinicians confirm BV using a combination of signs: the characteristic discharge, a vaginal pH above 4.5, the presence of “clue cells” (vaginal cells coated in bacteria visible under a microscope), and a fishy odor when the discharge is exposed to a chemical solution. Meeting three of these four criteria confirms the diagnosis.
Why It Matters Beyond Discomfort
BV is often treated as a minor nuisance, but untreated infections carry real risks. BV increases susceptibility to sexually transmitted infections, including HIV, because the disrupted vaginal lining offers less of a barrier to incoming pathogens. It also raises the risk of pelvic inflammatory disease, which can affect fertility.
During pregnancy, the stakes are higher. Research has consistently shown a roughly twofold increased risk of preterm delivery among pregnant women with BV. When BV is present in the first trimester, the risks climb further: one study found a nearly sevenfold increase in preterm delivery and a similar increase in premature rupture of the membranes. BV diagnosed early in pregnancy has also been linked to a three- to fivefold increased risk of miscarriage. These findings are why many providers screen for BV during prenatal care, particularly in women with a history of preterm birth.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV. Up to half of women treated successfully will have another episode within 12 months. The biofilm formed by Gardnerella vaginalis is a key reason. Standard antibiotic treatment kills the free-floating bacteria but often fails to fully penetrate the biofilm, allowing it to regrow once treatment ends.
Sexual transmission also plays a role in recurrence. If a male partner carries BV-associated bacteria, reinfection can happen with each sexual encounter, even after the woman has been treated. This is why the 2023 Indiana University findings are significant: they suggest that treating both partners could reduce recurrence, similar to how other STIs are managed. That approach isn’t yet standard practice, but it’s an active area of clinical discussion.
Avoiding douching, reducing exposure to vaginal irritants, and using condoms with new partners are the most practical steps to lower the chances of a first episode or a repeat one.