Bacterial vaginosis, or BV, is the most common vaginal infection during pregnancy, affecting roughly 12% to 49% of pregnant women depending on the population studied. It happens when the balance of bacteria in the vagina shifts: protective bacteria decline and other types multiply rapidly, sometimes by 100 to 1,000 times their normal levels. BV during pregnancy matters because it’s linked to a higher risk of preterm birth and other complications, but it’s treatable with medications considered safe at any stage of pregnancy.
What Happens in Your Body
A healthy vagina is dominated by beneficial bacteria called Lactobacillus, which keep the environment slightly acidic and crowd out harmful organisms. In BV, Lactobacillus populations drop sharply, and a mix of other bacteria takes over. The most common ones include Gardnerella, Prevotella, Mobiluncus, and Atopobium, though the exact mix varies from person to person.
This shift raises the vaginal pH above 4.5, creating an environment where these organisms thrive. During pregnancy, the specific type of Lactobacillus present seems to matter. Women with vaginal communities dominated by one particular strain (L. crispatus) have lower rates of preterm birth than those with other Lactobacillus types, or those who lose their Lactobacillus populations altogether.
How BV Feels and How It’s Found
Many pregnant women with BV have no symptoms at all. When symptoms do appear, the most common is a thin, milky-white or grayish discharge with a noticeable fishy smell. The odor is often stronger after sex. Unlike yeast infections, BV rarely causes significant itching or thick, clumpy discharge.
Diagnosis typically involves a clinical exam. Your provider looks for at least three of four signs: the characteristic thin discharge, a vaginal pH above 4.5, a fishy odor when a chemical solution is applied to a sample, and the presence of “clue cells” (vaginal cells coated in bacteria visible under a microscope). A vaginal Gram stain, which scores the bacterial pattern on a 0 to 10 scale, is the gold standard. A score of 7 or higher confirms BV. Newer molecular tests exist but are generally used only for women who have symptoms.
Current guidelines do not recommend routine BV screening for all pregnant women, even those at higher risk for preterm delivery. Testing is typically done when you report symptoms.
Why BV Matters During Pregnancy
BV is linked to several pregnancy complications, with preterm birth being the most studied. A large meta-analysis of more than 50,000 patients found that women with BV had about 60% higher odds of delivering preterm compared to women without it. Earlier research estimated the risk even higher, around double. The association is strongest for earlier preterm births: one study found that 22.7% of women with BV delivered before 34 weeks, compared to just 6.2% of those without it, nearly a four-fold difference.
BV has also been connected to premature rupture of membranes (when your water breaks too early) and miscarriage. Research on vaginal bacteria in women who miscarried during the first or second trimester consistently shows reduced Lactobacillus levels during or before the loss. Women whose membranes ruptured prematurely also showed depleted Lactobacillus and an overgrowth of other organisms.
Who Is More Likely to Get BV
BV prevalence varies significantly by race and ethnicity. In North American studies, Black and Hispanic women had prevalence rates of 33% and 31%, respectively, compared to 23% in White women and 11% in Asian women. The reasons behind these disparities are complex and likely involve a combination of differences in baseline vaginal microbiome composition, socioeconomic factors, and access to care. Having a new sexual partner, douching, and smoking also increase risk, whether or not you’re pregnant.
Treatment During Pregnancy
BV in pregnancy is treated with the same core medications used outside of pregnancy. The most common option is a seven-day course of oral metronidazole, taken twice daily. Vaginal options include metronidazole gel applied once daily for five days or clindamycin cream applied at bedtime for seven days. Oral clindamycin is available as an alternative.
Safety data on these medications during pregnancy is reassuring. Metronidazole does cross the placenta, but multiple large studies in pregnant women have found no evidence it causes birth defects or genetic damage in infants. Vaginal clindamycin was once flagged as a potential concern during pregnancy, but newer data shows it’s safe as well. One medication used for BV outside of pregnancy, tinidazole, should be avoided because animal studies suggest moderate risk and human data is limited.
Treatment courses typically last five to seven days. If you’re prescribed oral metronidazole, a metallic taste and nausea are common side effects. The vaginal formulations tend to cause fewer of these systemic effects.
Recurrence Is Common
One of the most frustrating aspects of BV is that it frequently comes back after treatment. This happens because antibiotics address the overgrowth of harmful bacteria but don’t necessarily restore a healthy Lactobacillus population. Your provider may recommend a follow-up visit about a month after treatment to check whether the infection has cleared, since treatment failures are common enough to warrant rechecking.
Probiotics containing Lactobacillus strains have shown some promise for restoring vaginal bacteria after BV treatment. Clinical trials using oral or vaginal Lactobacillus supplements found higher cure rates and fewer recurrences compared to placebo in several studies. However, results are inconsistent. Other trials found no significant difference between probiotics and placebo. The evidence is encouraging but not strong enough to recommend probiotics as a standalone treatment. If you’re interested in trying them alongside prescribed antibiotics, it’s worth discussing with your provider.
Reducing Your Risk
There’s no guaranteed way to prevent BV, but a few practices help maintain a healthy vaginal environment. Avoid douching, which strips away protective bacteria. Use unscented soaps and avoid inserting any fragranced products vaginally. Wearing cotton underwear and changing out of wet clothing promptly can also help, though these measures are more about general vaginal health than BV-specific prevention.
If you notice a change in vaginal discharge during pregnancy, particularly a thin discharge with a fishy odor, bring it up at your next prenatal visit or call your provider’s office. BV is straightforward to diagnose and treat, and addressing it early reduces the chance of complications.