Bacterial vaginosis (BV) develops when the natural balance of bacteria inside the vagina shifts, allowing certain organisms to multiply out of control. It’s the most common vaginal infection in women of reproductive age, and it doesn’t have a single, simple cause. Instead, several everyday factors can tip the balance, from sexual activity to hygiene habits to the products you use.
What Happens Inside the Vagina
A healthy vagina is home to a community of bacteria, and the dominant players are usually Lactobacillus species. These bacteria produce hydrogen peroxide and other antimicrobial compounds that keep the vaginal environment acidic, typically at a pH below 4.5. That acidity acts like a shield: it suppresses harmful organisms and prevents them from gaining a foothold.
BV occurs when Lactobacillus populations drop and anaerobic bacteria (organisms that thrive without oxygen) take over. The most common culprits include Gardnerella vaginalis, Atopobium vaginae, and Mycoplasma hominis. Once these bacteria multiply, vaginal pH rises above 4.5, the protective acidic environment weakens, and the overgrowth feeds on itself. This shift from a Lactobacillus-dominated environment to a mixed anaerobic community is the core of what BV actually is.
Sexual Activity and BV
BV is not classified as a sexually transmitted infection, but it is closely linked to sexual activity. Researchers believe sex changes the bacterial environment inside the vagina in ways that make overgrowth more likely. A few specific mechanisms help explain why.
Semen is significantly more alkaline than the vagina, with a pH between 7.2 and 7.8, compared to the vagina’s normal range below 4.5. When semen enters the vagina, it temporarily raises the pH, creating conditions that favor anaerobic bacteria over protective Lactobacillus. This doesn’t mean every instance of unprotected sex causes BV, but repeated exposure to semen without condom use increases the cumulative risk.
Having a new sexual partner or multiple partners also raises BV risk. Each partner introduces a different set of bacteria, and the vaginal microbiome needs time to adapt. Women who have sex with women can pass BV-associated bacteria between partners as well. Condom use reduces risk because it limits the vaginal microbiome’s exposure to outside bacteria and to the alkaline effect of semen.
Douching and Hygiene Products
Douching is one of the most well-established risk factors for BV. The practice flushes out both harmful and protective bacteria, stripping away the Lactobacillus populations that maintain a healthy pH. The CDC lists not douching as a basic prevention step for BV. The vagina is self-cleaning, and introducing water, vinegar, or commercial douching solutions disrupts the chemistry it works to maintain on its own.
Scented feminine products carry a similar risk. Fragrance, chlorine, titanium dioxide, and essential oils like lavender, mint, and rose can all irritate vaginal tissue and disrupt pH levels when they come into contact with the vaginal area. This applies to scented tampons, pads, sprays, and washes. Even “natural” ingredients like aloe vera or essential oils can cause the same disruption as synthetic fragrances. Unscented, minimal-ingredient products are a safer choice for anything that touches the vulva or vaginal area.
Other Factors That Raise Your Risk
Beyond sex and hygiene habits, a few other factors make BV more likely:
- Antibiotics: Taking antibiotics for any infection can reduce Lactobacillus levels along with the targeted bacteria, leaving the vagina vulnerable to anaerobic overgrowth.
- Smoking: Cigarette smoking is associated with higher BV rates, possibly because it affects the immune response in vaginal tissue.
- IUDs: Some women develop BV after IUD insertion, though the connection isn’t fully understood.
- Hormonal changes: Fluctuations in estrogen during menstruation, pregnancy, or menopause can shift vaginal pH and alter the bacterial community.
BV also has a frustrating tendency to recur. Up to half of women treated for BV experience another episode within 12 months, often because the underlying conditions that disrupted the microbiome in the first place haven’t changed.
How BV Feels and How It’s Diagnosed
Many women with BV notice a thin, grayish-white discharge with a distinctive fishy smell, especially after sex. The odor comes from compounds produced by the anaerobic bacteria. Some women also experience mild itching or irritation, though BV is often less uncomfortable than a yeast infection. About half of women with BV have no noticeable symptoms at all.
Diagnosis typically involves a pelvic exam and a few quick tests. Your provider checks whether vaginal pH is above 4.5, looks at a sample under a microscope for “clue cells” (vaginal cells coated in bacteria), and may test for the characteristic fishy odor. At least three of these signs together confirm a BV diagnosis.
Why Treating BV Matters
Left untreated, BV isn’t just an annoyance. It increases vulnerability to sexually transmitted infections, including HIV, chlamydia, and gonorrhea, because the loss of protective Lactobacillus weakens the vagina’s natural defenses. BV also raises the risk of infection after gynecological procedures.
During pregnancy, the stakes are higher. A large meta-analysis found that pregnant women with BV face about 60% greater odds of preterm birth compared to women without BV. Preterm birth was the most common complication, followed by low birth weight and other newborn health issues. Treatment during pregnancy can reduce these risks, which is why providers often screen for BV in pregnant women who have symptoms or a history of preterm delivery.
Lowering Your Risk
You can’t control every factor, but several practical steps reduce the likelihood of BV. Using condoms consistently limits exposure to outside bacteria and semen’s pH-raising effect. Avoiding douching allows the vagina’s natural cleaning process to work. Choosing unscented products for anything near the vulva or vagina keeps chemical irritants away from sensitive tissue. Wearing breathable cotton underwear and changing out of wet swimsuits or workout clothes promptly can also help maintain a stable vaginal environment.
If you’ve had BV before and it keeps coming back, pay attention to what changed before each episode. A new partner, a new product, a round of antibiotics: identifying your personal triggers gives you the best shot at breaking the cycle.