Bacterial Vaginosis (BV) is a common condition resulting from an imbalance of the naturally occurring bacteria in the vagina. This shift involves a decrease in beneficial Lactobacilli and an overgrowth of various anaerobic organisms, such as Gardnerella vaginalis. While BV is not considered a sexually transmitted infection, it is associated with sexual activity and is the most frequent cause of abnormal vaginal discharge in women of reproductive age. During pregnancy, this imbalance is significant because it has been linked to several adverse outcomes for the mother and the developing fetus.
Prevalence and Contributing Factors in Pregnancy
Bacterial Vaginosis is remarkably common among pregnant women, with prevalence estimates typically ranging from 10% to 30% globally. The Centers for Disease Control and Prevention (CDC) estimates that approximately one million pregnant women in the United States contract BV each year. This high incidence makes BV one of the most prevalent vaginal infections encountered during pregnancy.
Physiological changes during pregnancy contribute to increased susceptibility to BV. Pregnancy naturally elevates circulating estrogen, which increases glycogen production in vaginal cells. While this glycogen feeds acid-producing Lactobacilli that maintain a low vaginal pH, hormonal shifts can still disrupt the delicate microbial balance.
The hormonal milieu of pregnancy can also alter the local immune response and the composition of cervical mucus, potentially making the environment more hospitable for the overgrowth of anaerobic bacteria. When the protective Lactobacilli are displaced, the vaginal pH rises above its normal acidic level of 4.5, creating favorable conditions for the BV-associated bacteria to flourish. This dysbiosis can be persistent.
Specific Risks to the Mother and Fetus
The primary concern regarding BV during pregnancy is the risk of ascending infection, where bacterial overgrowth moves from the lower genital tract into the uterus. This ascent is facilitated by bacterial enzymes breaking down the protective cervical mucus barrier. This process causes inflammation in the upper reproductive tract, which is thought to be the direct cause of several serious obstetric complications.
BV is a well-established risk factor for Preterm Birth (PTB), defined as delivery before 37 weeks of gestation, and Preterm Premature Rupture of Membranes (PPROM). The bacteria associated with BV, such as Prevotella bivia and Mobiluncus, produce enzymes like proteases and phospholipases that weaken the fetal membranes and trigger the onset of labor.
The inflammatory cytokines released during the infection weaken the chorioamniotic membranes, leading to their premature rupture. Beyond prematurity, BV is also linked to a higher incidence of Low Birth Weight (LBW) and Intrauterine Growth Restriction (IUGR). These neonatal outcomes are associated with long-term health and developmental challenges for the infant.
Maternal complications are also a significant risk, including postpartum endometritis, which is an infection of the uterine lining following delivery. BV has also been associated with an increased risk of chorioamnionitis, a severe bacterial infection of the placental membranes and amniotic fluid. The risk of these complications is highest for women who have a history of a previous preterm birth.
Diagnosis and Treatment Protocols
Diagnosis of Bacterial Vaginosis in pregnancy typically relies on clinical criteria or laboratory testing, as many cases are asymptomatic. The gold standard for laboratory diagnosis is the Nugent scoring system, which involves a Gram stain of the vaginal discharge to quantify the different bacterial types present and assess the degree of microbial shift. Amsel’s clinical criteria offer a rapid point-of-care diagnosis, requiring at least three of four findings:
- Thin, homogenous discharge.
- A vaginal pH greater than 4.5.
- The presence of “clue cells” on microscopy.
- A positive “whiff test” indicating a fishy odor after adding potassium hydroxide.
For symptomatic pregnant women, treatment is recommended to alleviate discomfort and reduce the risk of adverse outcomes. The standard management involves systemic antibiotics, such as oral metronidazole or oral clindamycin, which are considered safe during pregnancy. Oral therapy is preferred over topical creams because systemic treatment is more effective at clearing bacteria that have ascended into the upper genital tract.
Current guidelines, including those from the CDC, suggest that routine screening and treatment of asymptomatic pregnant women is not recommended for the sole purpose of preventing preterm birth in the general population. However, treatment is often considered for asymptomatic women who are at a higher risk of preterm birth, particularly those with a history of spontaneous preterm delivery. Given the high rate of recurrence for BV, even after successful treatment, adherence to the full antibiotic course and a follow-up test-of-cure may be necessary to ensure the infection has been fully eradicated.