Bacterial vaginosis (BV) during pregnancy does carry real risks, particularly for preterm birth. Women with BV have roughly two to three times the risk of delivering early compared to women without it. The good news: BV is treatable during pregnancy, and getting it addressed when symptoms appear reduces the chance of complications.
How BV Affects Pregnancy Outcomes
The most significant risk is preterm birth. In one study, 50% of pregnant women with BV delivered preterm, compared to 21% of women without it. The risk of very early delivery (before 34 weeks) was especially pronounced: 22.7% in the BV group versus 6.2% in the BV-negative group. Multiple studies have confirmed this pattern, consistently finding a two- to nearly three-fold increase in preterm delivery risk.
Birth weight is also affected. Babies born to mothers with BV had a median birth weight of about 2,450 grams (roughly 5.4 pounds), compared to 2,950 grams (6.5 pounds) for babies born to mothers without BV. That’s more than a full pound of difference, which matters for a newborn’s health in those first days and weeks.
Beyond preterm birth and low birth weight, BV is associated with premature rupture of membranes (your water breaking too early), infection of the amniotic fluid, and uterine infections after delivery.
Why BV Creates These Risks
BV isn’t caused by a single invading organism. It’s a shift in the vaginal microbiome where the normally dominant beneficial bacteria get crowded out by a mix of other species. During pregnancy, this imbalance opens the door in several ways. First, it makes it easier for harmful bacteria to travel upward from the vagina toward the uterus and amniotic sac. Second, BV-associated bacteria produce enzymes that weaken the body’s local immune defenses. Third, the bacterial byproducts trigger the release of inflammatory compounds, including prostaglandins, which are the same chemicals involved in starting labor contractions. This inflammatory cascade is what drives the link between BV and preterm delivery.
How Common BV Is in Pregnancy
BV prevalence in pregnant women ranges from about 6% to 32%, depending on the population. In the United States and parts of Africa, rates tend to fall in the 15% to 30% range. In European studies, rates are generally lower, around 6% to 11%. These differences reflect a mix of genetic, environmental, and socioeconomic factors rather than any single cause.
Recognizing BV During Pregnancy
Pregnancy already comes with increased vaginal discharge, which makes it easy to dismiss BV symptoms as normal. The key differences: BV discharge is typically thin, white or grey, and has a strong fishy odor, especially after sex. Normal pregnancy discharge tends to be thicker, milky white, and mild-smelling. About half of women with BV have no noticeable symptoms at all, which is part of what makes it tricky.
If you notice a change in your discharge, particularly a new or worsening fishy smell, it’s worth mentioning at your next prenatal visit rather than assuming it’s a normal pregnancy change.
How BV Is Diagnosed
Your provider can diagnose BV during a routine office visit. The most common method uses a set of clinical signs: the appearance and consistency of discharge, whether the vaginal pH is above 4.5, the presence of “clue cells” (vaginal cells coated in bacteria visible under a microscope), and whether the discharge produces a fishy odor when tested. Meeting at least three of these four criteria points to BV.
A vaginal swab examined under a microscope (Gram stain) is considered the gold standard and assigns a numerical score based on the types of bacteria present. Newer molecular tests are also available but are currently recommended only for women who have symptoms, since their accuracy in symptom-free women isn’t well established.
Treatment During Pregnancy
The CDC recommends treating all pregnant women who have symptomatic BV. Antibiotics are the standard treatment, and the options used during pregnancy have well-established safety profiles. Treatment typically lasts about seven days and can be taken orally or applied vaginally, depending on your provider’s recommendation and your preference.
One important nuance: routine screening for BV in pregnant women who have no symptoms is not currently recommended, even for those considered high-risk for preterm delivery. A large multicenter trial found that treating asymptomatic BV in pregnancy did not reduce adverse outcomes. This means the current approach is symptom-driven: if you have signs of BV, treatment is clearly beneficial, but screening everyone “just in case” hasn’t proven helpful.
Can Probiotics Prevent BV in Pregnancy?
The idea of using probiotics to maintain vaginal health during pregnancy is appealing, but the evidence is disappointing. In a double-blind, placebo-controlled trial of pregnant women, oral probiotics did not reduce the rate of vaginal infections compared to placebo. The infection rate was essentially identical: 29% in the probiotic group and 27% in the placebo group. Researchers also found that bacteria from the probiotic capsules never actually colonized the vagina, meaning they didn’t reach the area they were supposed to protect. At this point, probiotics are not a reliable strategy for preventing BV during pregnancy.
What This Means for You
BV during pregnancy is a real concern, not because it’s an emergency, but because untreated symptomatic BV roughly doubles or triples your risk of delivering too early. The practical takeaway is straightforward: pay attention to changes in vaginal discharge, bring up anything unusual with your prenatal provider, and follow through on treatment if BV is diagnosed. The condition responds well to antibiotics, and addressing it promptly is the most effective way to reduce its impact on your pregnancy.