Bacterial vaginosis (BV) happens when the balance of bacteria in the vagina shifts. Protective bacteria called lactobacilli decline, and a mix of other bacteria overgrow in their place. It affects roughly 23 to 29% of women of reproductive age worldwide, making it the most common vaginal condition. The causes aren’t always obvious, and several factors can trigger this shift independently or together.
How the Vaginal Microbiome Shifts
A healthy vagina is dominated by lactobacilli, large rod-shaped bacteria that produce hydrogen peroxide and lactic acid. These compounds keep the vaginal pH acidic, typically below 4.5, which suppresses the growth of other organisms. When lactobacilli decline for any reason, that acidic environment weakens, and anaerobic bacteria (species that thrive without oxygen) multiply rapidly to fill the gap.
The bacterium most consistently found in BV is Gardnerella vaginalis, detected in about 96% of women with the condition. But BV is not a single-germ infection. It’s a collaborative takeover by multiple species, including Prevotella, Mobiluncus, Atopobium vaginae, and others. Gardnerella plays a central role because it builds a biofilm, a sticky, structured community of microbes that adheres to the vaginal lining. This biofilm shields the bacteria inside it from the body’s defenses and even from antibiotics, which helps explain why BV so often comes back. Up to 66% of women experience a recurrence within a year of treatment.
Sexual Activity and New Partners
BV is not classified as a sexually transmitted infection, but sexual activity is one of the strongest risk factors. Having a new sexual partner, or multiple partners, significantly increases the chance of developing it. BV rarely affects people who have never had sex.
The connection likely involves the introduction of unfamiliar bacteria during intercourse, which can destabilize the existing microbial community. Unprotected sex appears to carry higher risk; condom use is associated with lower BV rates. Despite this link, treating a male sexual partner has not been shown to prevent BV in most research, though recent guidance from the American College of Obstetricians and Gynecologists now recommends concurrent partner treatment specifically for recurrent cases.
Women who have sex with women also experience BV at notable rates, and partners often share the same BV-associated bacteria, reinforcing the idea that intimate contact facilitates microbial exchange regardless of the type of sexual activity.
Douching and Hygiene Products
Douching is one of the most well-documented triggers for BV. The practice flushes out the hydrogen peroxide-producing lactobacilli that maintain vaginal acidity. Once that protective system is disrupted, anaerobic bacteria can overgrow quickly. The American Academy of Family Physicians has noted that douching directly disturbs the vaginal protective systems based on hydrogen peroxide and lactobacilli, permitting overgrowth of the very bacteria responsible for BV.
Scented soaps, bubble baths, vaginal deodorants, and other fragranced products applied in or around the vagina can have a similar effect. They alter pH and irritate the vaginal lining, creating conditions that favor the wrong bacteria. The vagina is self-cleaning, and most gynecologists recommend washing only the external area with plain water or a mild, unscented cleanser.
Hormonal Changes
Estrogen plays a quiet but critical role in vaginal health. It promotes the growth of vaginal lining cells that produce glycogen, a sugar that lactobacilli feed on. When estrogen drops, there’s less fuel for lactobacilli, and vaginal pH drifts from acidic toward alkaline. That shift gives Gardnerella and other BV-associated bacteria room to overgrow.
This is why certain life stages carry higher BV risk. Perimenopause and menopause bring sustained drops in estrogen and progesterone, making BV more likely in older women. Hormonal fluctuations during the menstrual cycle can also create temporary vulnerability. Some women notice BV symptoms appearing or worsening right after their period, when vaginal pH is naturally higher due to the alkaline nature of menstrual blood.
Antibiotics for Other Conditions
Systemic antibiotics taken for unrelated infections, such as a urinary tract infection, sinus infection, or strep throat, don’t discriminate between harmful and beneficial bacteria. They can kill off vaginal lactobacilli as collateral damage, leaving the vagina temporarily unprotected. This creates an opening for anaerobic bacteria to establish themselves. The result can be a new case of BV that appears to come out of nowhere, days or weeks after finishing an antibiotic course for something entirely different.
Genetic and Immune Factors
Not everyone exposed to the same risk factors develops BV, and researchers have been investigating why. Racial disparities in BV rates persist even after accounting for behavioral differences like sexual practices and douching, suggesting that genetics play a role. Women of African descent, for example, have consistently higher BV prevalence across studies.
Research on Kenyan women identified associations between vaginal microbiome composition and genes involved in the innate immune system, particularly toll-like receptors and cytokine production pathways. These are components of the body’s first-line defense system that help determine which bacteria are tolerated and which are fought off. While no single gene has been confirmed as a definitive BV risk factor, the pattern suggests that some women’s immune systems are less effective at maintaining lactobacilli dominance, making them inherently more susceptible to the microbial shift that causes BV.
Smoking
Cigarette smoking is consistently associated with higher BV rates. Nicotine and other tobacco metabolites concentrate in cervical mucus and appear to be toxic to lactobacilli. Smoking also suppresses local immune function in the vaginal tissue, reducing the body’s ability to keep anaerobic bacteria in check. Women who smoke are roughly twice as likely to develop BV compared to nonsmokers, and the association holds even when other risk factors are controlled for.
IUDs and Contraceptive Choices
Intrauterine devices, particularly copper IUDs, have been linked to increased BV risk in some studies. The mechanism likely involves changes to menstrual bleeding patterns and local inflammation that can alter the vaginal environment. Hormonal contraceptives, on the other hand, tend to have a neutral or even protective effect, possibly because they stabilize estrogen levels and reduce menstrual flow.
Why BV Keeps Coming Back
The biofilm that Gardnerella builds on the vaginal wall is central to understanding recurrence. Studies have shown this biofilm can survive exposure to hydrogen peroxide, lactic acid, and even high concentrations of antibiotics. Treatment may clear the symptoms and reduce bacterial counts, but if the biofilm isn’t fully eradicated, it can serve as a reservoir from which the infection rebounds.
Ongoing exposure to the same triggers, whether that’s a sexual partner carrying BV-associated bacteria, continued douching, or hormonal fluctuations, also contributes. For many women, recurrent BV isn’t a new infection each time but a reactivation of one that never fully resolved. This is why the recurrence rate is so high: two out of three women will see BV return within 12 months of their initial treatment.
BV During Pregnancy
BV carries particular significance during pregnancy. A meta-analysis of over 50,000 patients found that BV is associated with a 60% increased odds of preterm birth. Earlier analyses placed that figure even higher, with some estimates showing a twofold increase in preterm delivery risk. BV during pregnancy has also been linked to late miscarriage and low birth weight. Pregnant women who notice the characteristic thin, grayish-white discharge or fishy odor should bring it up with their provider, as screening and treatment during pregnancy can reduce these risks.