Bacterial vaginosis is one of the most common vaginal infections during pregnancy, affecting an estimated 10 to 35 percent of pregnant women. Some studies have found rates as high as 50 percent depending on the population studied. If you’re pregnant and dealing with BV, or wondering whether that unusual discharge is something to worry about, you’re far from alone.
How Common BV Is Across Different Groups
The prevalence of BV in pregnancy varies significantly depending on demographics. In large studies, rates range from about 10 percent among privately insured patients to 35 percent among women with lower incomes or less access to healthcare. Black women face a 2.0 to 2.5 times higher risk of BV during pregnancy compared to white women, a disparity documented in the Vaginal Infections and Prematurity Study. The reasons for this gap aren’t fully understood but likely involve a combination of differences in baseline vaginal microbiome composition, stress, and healthcare access rather than any single biological factor.
Interestingly, no large studies have tracked exactly how BV prevalence shifts across trimesters. Researchers know the vaginal environment changes dramatically throughout pregnancy, but whether BV is more likely to appear at a particular stage remains an open question.
Why Pregnancy Changes Your Vaginal Environment
Pregnancy reshapes the vaginal ecosystem in several ways that can tip the balance toward BV. Estrogen and progesterone levels during pregnancy climb to roughly double what they are during a normal menstrual cycle. This hormonal surge causes extra glycogen to build up in vaginal tissue, which feeds a wider variety of bacteria and increases overall microbial diversity.
At the same time, your immune system partially suppresses itself during early pregnancy to allow the embryo to implant safely. This immune shift triggers low-grade inflammation across mucosal surfaces, including the vagina. The combination of immune suppression and hormonal changes can push the vaginal microbiome away from its usual protective, lactobacillus-heavy state toward the kind of high-diversity bacterial mix that characterizes BV. This is especially pronounced in the first trimester, when immune suppression is most dramatic.
Telling BV Apart From Normal Pregnancy Discharge
Pregnancy naturally increases vaginal discharge. Normal pregnancy discharge, called leukorrhea, is clear or milky white, thin, and has only a mild scent. This is completely expected and not a sign of infection.
BV discharge looks and smells different. It tends to be gray in color and produces a strong fishy odor that often becomes more noticeable after sex. If your discharge has shifted from clear or white to grayish with a distinct smell, that pattern points toward BV rather than the normal increase in discharge that comes with pregnancy. Some women with BV also notice irritation or burning, though many have no symptoms at all beyond the discharge change.
Why BV During Pregnancy Matters
BV isn’t just uncomfortable. It carries real risks for pregnancy outcomes. The most significant concern is preterm birth. A recent systematic review and meta-analysis found that BV increases the odds of preterm delivery by about 60 percent overall, and earlier research found the risk could more than double. The leading theory is that bacteria from the vagina can travel upward to the fetal membranes, potentially triggering preterm labor or causing the membranes to rupture early.
Babies born prematurely as a result face their own set of challenges, including low birth weight and breathing difficulties. BV has also been associated with infection of the amniotic fluid and uterine infection after delivery. These risks are why symptomatic BV during pregnancy is treated rather than watched.
Screening: Who Gets Tested and When
You might assume every pregnant woman would be screened for BV given these risks, but current guidelines don’t support universal screening. The U.S. Preventive Services Task Force recommends against screening pregnant women who aren’t at increased risk for preterm delivery, concluding with moderate certainty that screening this group provides no net benefit in preventing preterm birth. For women who are at higher risk of preterm delivery (such as those with a history of preterm birth), the evidence is conflicting, and the task force considers it insufficient to make a firm recommendation either way.
Clinical trials targeting general obstetric populations have consistently found no benefit from screening and treating asymptomatic BV. This means testing is typically reserved for women who report symptoms or who have specific risk factors. If you notice the gray, fishy-smelling discharge described above, bring it up with your provider rather than waiting for routine screening to catch it.
How BV Is Treated During Pregnancy
Treatment is recommended for all pregnant women with symptomatic BV. The CDC confirms that the standard antibiotics used for BV are safe during pregnancy. Oral and vaginal options are both available, and treatment courses typically last five to seven days. Despite older concerns, the most commonly prescribed antibiotic for BV crosses the placenta but has shown no evidence of birth defects or genetic harm to infants across multiple large studies. Vaginal antibiotic creams, once thought to pose risks during pregnancy, have also been confirmed safe by more recent data.
The bigger challenge with BV treatment is recurrence. Across all women (not just pregnant women), 50 to 80 percent experience BV again within six to twelve months of completing antibiotics. This high recurrence rate means that a single course of treatment doesn’t always resolve the issue for good, and some women find themselves dealing with repeat episodes throughout pregnancy.
Reducing Your Risk of BV and Recurrence
Probiotic research during pregnancy has shown some promising results. Two specific probiotic strains, Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14, have been studied in pregnant women and found to help restore normal vaginal bacteria and acidic pH. In one clinical trial, pregnant women who took these probiotics before 20 weeks of gestation had roughly half the rate of spontaneous preterm delivery before 34 weeks compared to the placebo group (about 3 percent versus 6 percent). These strains have been granted “Qualified Presumption of Safety” status by the European Food Safety Authority and work by competing with harmful bacteria for space and nutrients on vaginal tissue while producing lactic acid and hydrogen peroxide to maintain acidity.
Beyond probiotics, general practices that support vaginal health apply during pregnancy too. Avoiding douching, wearing breathable cotton underwear, and not using scented products in the vaginal area all help maintain the bacterial balance that keeps BV at bay. None of these measures guarantee prevention, but they reduce the environmental disruptions that allow BV-associated bacteria to flourish.