Bacterial vaginosis (BV) happens when the natural balance of bacteria in the vagina shifts. Normally, beneficial bacteria called lactobacilli dominate the vaginal environment, keeping it acidic and inhospitable to harmful microbes. When something disrupts that balance, other bacteria multiply rapidly and take over. The result is BV, the most common vaginal infection in women of reproductive age.
How the Vagina Normally Protects Itself
A healthy vagina maintains a pH between 3.8 and 4.5, roughly as acidic as a tomato. That acidity is the work of lactobacilli, which ferment sugars and produce lactic acid as a byproduct. This low pH is directly inhibitory to the growth of many potential colonizers, essentially creating a chemical barrier that keeps harmful bacteria from gaining a foothold.
Lactobacilli also produce hydrogen peroxide, which acts as a second line of defense. The hydrogen peroxide generates oxidizing molecules that damage the DNA of unwanted microbes. This effect is amplified by the acidic environment itself, making the combination of lactic acid and hydrogen peroxide particularly effective. When lactobacilli are thriving, the vaginal ecosystem is remarkably self-regulating.
What Feeds the Good Bacteria
Estrogen plays a central, often overlooked role in this system. It signals the cells lining the vaginal wall to stockpile glycogen, a form of stored sugar. Lactobacilli break down that glycogen into lactic acid, which keeps the pH low. Estrogen also directly stimulates the vaginal lining to secrete acid into the vaginal canal, reinforcing the effect.
This is why BV risk changes across a woman’s life. During reproductive years, estrogen levels are high and glycogen is abundant. After menopause, estrogen drops, glycogen decreases, and the pH rises above 4.5. The same shift happens temporarily just before a period, when estrogen dips. Any time estrogen is lower than usual, the food supply for lactobacilli shrinks, and the environment becomes more vulnerable to bacterial disruption.
Common Triggers That Shift the Balance
Sexual Activity
Sex is one of the most common triggers, though BV is not a sexually transmitted infection. Semen has a pH between 7.2 and 7.8, which is significantly more alkaline than the vaginal environment. When semen enters the vagina, it temporarily raises the pH, creating a window where harmful bacteria can multiply more easily. New sexual partners or multiple partners increase exposure to unfamiliar bacteria, which compounds the effect. Using condoms reduces this risk because they prevent semen from contacting the vaginal walls.
Douching
Douching is one of the strongest modifiable risk factors for BV. Studies show women who douche are about 2.3 times more likely to develop genital infections than women who don’t, and their risk of abnormal vaginal discharge increases nearly fourfold. Even douching with something seemingly harmless like saline can reduce the number and diversity of vaginal bacteria within 10 minutes. Douching with antiseptic solutions is worse: it kills lactobacilli along with everything else, then pathogenic organisms rebound faster and repress the beneficial bacteria that were there before. The vagina is self-cleaning, and douching undermines the very system designed to keep it healthy.
Antibiotics
Antibiotics taken for any reason, not just vaginal infections, can deplete lactobacilli as collateral damage. Once the protective bacteria are reduced, opportunistic species fill the gap. This is one reason BV sometimes appears shortly after a course of antibiotics for an unrelated condition like a sinus infection or urinary tract infection.
Smoking
Smoking is consistently linked to higher BV rates. The chemicals in cigarette smoke reduce lactobacillus populations in the vaginal lining, though the exact pathway isn’t fully mapped. Women who smoke are significantly more likely to have recurrent episodes.
Why BV Keeps Coming Back
Recurrence is the defining frustration of BV. Within 6 to 12 months of finishing antibiotic treatment, 50% to 80% of women experience another episode. That’s an extraordinarily high recurrence rate, and it points to something important about the condition: antibiotics kill the overgrown bacteria, but they don’t rebuild the lactobacillus-dominant ecosystem that was lost. If the underlying disruption, whether it’s a shift in pH, ongoing exposure to triggers, or a depleted lactobacillus population, isn’t addressed, the same imbalance tends to reassert itself.
Some researchers believe that harmful bacteria form a protective film on the vaginal lining, called a biofilm, that antibiotics can’t fully penetrate. The surviving bacteria within that film then repopulate after treatment ends. This is an active area of investigation and helps explain why standard treatment so often falls short.
How BV Affects Pregnancy
BV during pregnancy carries real risks. In one study, the rate of preterm birth before 34 weeks was nearly four times higher in women with BV compared to those without (22.7% versus 6.2%). Babies born to mothers with BV had lower birth weights, were more than twice as likely to need intensive care admission, and had significantly higher rates of respiratory distress. More than half of women with BV in the study showed signs of infection in the placental tissue. These outcomes held even among women who received treatment, suggesting that the inflammation caused by BV can do damage before antibiotics take effect.
What BV Is Not
BV is not caused by poor hygiene. In fact, excessive cleaning, especially with fragranced soaps or internal washing, makes it more likely. It’s also not a sexually transmitted infection in the traditional sense, though sexual activity is a trigger. Women who have never been sexually active can develop BV, and it occurs in women who have sex exclusively with other women. The condition reflects an ecological collapse within a complex microbial community, not contamination from an outside pathogen. Understanding this distinction matters because it reframes prevention: the goal isn’t to sterilize the vagina but to support the bacteria already doing the protective work.