Bacterial vaginosis (BV) during pregnancy is caused by a shift in the vaginal microbiome: protective bacteria decline, and a mix of anaerobic organisms overgrows in their place. Between 10 and 35 percent of pregnant women develop BV, with rates reaching as high as 50 percent in some populations. Pregnancy itself creates the conditions for this shift, but several behavioral and biological factors raise the risk further.
How the Vaginal Microbiome Shifts During Pregnancy
In a healthy vagina, Lactobacillus bacteria dominate. They produce lactic acid that keeps the environment acidic, typically below a pH of 4.5, which suppresses the growth of harmful organisms. BV develops when Lactobacillus populations drop and a diverse community of anaerobic bacteria fills the gap. This isn’t a single-germ infection. It’s a broad ecosystem disruption, which is why BV behaves differently from a standard infection and can be stubborn to resolve.
Pregnancy accelerates this process. Rising levels of estrogen and progesterone alter vaginal pH and change the chemical environment that Lactobacillus depends on. The result is a vagina that’s more hospitable to the anaerobic bacteria associated with BV. Research on vaginal microbial communities shows that Lactobacillus loses its normal network connections with other bacteria during early and late pregnancy, meaning the usual checks and balances that keep the ecosystem stable can weaken at exactly the wrong time.
Risk Factors That Make BV More Likely
Hormonal changes set the stage, but certain habits and circumstances push the odds higher. The CDC identifies three main behavioral risk factors: douching, not using condoms, and having new or multiple sexual partners. All of these disrupt the balance of vaginal bacteria in ways that favor anaerobic overgrowth.
Douching is especially relevant because many women increase hygiene practices during pregnancy, sometimes introducing products that strip away protective Lactobacillus. Even water-only douching can alter pH enough to trigger a shift. Beyond behavior, demographic factors also play a role. Higher rates of BV during pregnancy have been documented among Black women, women with lower incomes, women who smoke, those with a higher BMI, and women with a history of sexually transmitted infections. These patterns likely reflect a combination of environmental exposures, stress, and access to care rather than any single biological cause.
Why BV Matters More During Pregnancy
BV in pregnancy isn’t just uncomfortable. It has been associated with preterm delivery, early miscarriage, infection of the uterine lining after birth, and low birth weight. The mechanism behind these complications is becoming clearer: the anaerobic bacteria involved in BV produce enzymes, including proteases and collagenases, that can break down proteins in the fetal membranes. In simpler terms, these bacteria release substances that weaken the sac surrounding the baby, which can lead to premature rupture of membranes and trigger early labor.
This inflammatory cascade also makes the uterine environment more vulnerable to infection. Bacteria that would normally stay in the vagina can travel upward when the cervical barrier is compromised, increasing the risk of intra-amniotic infection. These risks are why symptomatic BV during pregnancy is always treated, not monitored.
Recognizing BV Symptoms
Many pregnant women with BV have no symptoms at all, which is part of what makes it tricky. When symptoms do appear, the most common signs include a thin, milky vaginal discharge that coats the vaginal walls and a noticeable fishy odor, particularly after sex. The discharge tends to be homogeneous rather than clumpy (which would suggest a yeast infection instead).
During a prenatal visit, your provider can check for BV using a few straightforward tests: examining the discharge under a microscope for “clue cells” (vaginal cells coated in bacteria), testing whether vaginal fluid pH is above 4.5, or sending a sample for a Gram stain. Newer nucleic acid tests can also detect specific BV-associated bacteria with high accuracy. If you notice a change in discharge or odor during pregnancy, mention it at your next appointment rather than assuming it’s a normal pregnancy change.
How BV Is Treated During Pregnancy
The CDC recommends treating all symptomatic BV in pregnant women because of the link to adverse outcomes. The standard options are the same ones used outside of pregnancy. Most commonly, you’ll be prescribed a seven-day course of oral metronidazole or a vaginal cream (either metronidazole gel or clindamycin cream). Oral clindamycin is also available as an alternative.
Safety data on these medications during pregnancy is reassuring. Multiple studies, including cross-sectional, case-control, and cohort designs, have found no evidence that metronidazole causes birth defects or genetic damage in infants. Clindamycin was once flagged for possible adverse newborn outcomes, but more recent data shows it is safe for use during pregnancy. One medication to avoid is tinidazole: animal studies suggest moderate risk, and human data is too limited to confirm safety.
Recurrence After Treatment
One of the most frustrating aspects of BV, pregnant or not, is how often it comes back. Across all women, BV recurs in 15 to 50 percent of cases within one to twelve months after successful treatment. During pregnancy, this is particularly concerning because the hormonal environment that contributed to the first episode hasn’t changed. Unfortunately, no specific guidelines exist for managing BV recurrence during pregnancy or for preventive treatment after a first episode resolves.
If BV returns during your pregnancy, your provider will likely prescribe another course of the same or a different antibiotic. Practical steps that may help reduce recurrence include avoiding douching entirely, using condoms if you’re sexually active during pregnancy, and skipping scented soaps or sprays in the vaginal area. None of these are guaranteed to prevent a recurrence, but they reduce the disruptions that allow anaerobic bacteria to regain a foothold.