Does bacterial vaginosis (BV) increase the risk of preterm labor?

Bacterial Vaginosis and Preterm Labor Risk

Yes, bacterial vaginosis (BV) significantly increases the risk of preterm labor, with studies showing a relative risk ranging from 1.4 to 6.9 for preterm delivery. 1

Association Between BV and Adverse Pregnancy Outcomes

• BV is characterized by an imbalance in vaginal bacterial flora with decreased Lactobacilli and increased Gardnerella vaginalis, Mycoplasma, and anaerobic bacteria 1
• Observational studies consistently demonstrate associations between BV and:
  • Preterm delivery (relative risk 1.4-6.9) 1
  • Preterm premature rupture of membranes (relative risk 2.0-7.3) 1
  • Preterm labor (relative risk 2.0-2.6) 1
  • Spontaneous abortion (relative risk 1.3-2.0) 1
• The most recent meta-analysis (2023) confirms BV is associated with approximately two-fold increased risk of preterm birth (OR 1.79,95% CI 1.32-2.43) 2

Pathophysiology

• BV involves replacement of normal hydrogen peroxide-producing Lactobacillus with high concentrations of aerobic and anaerobic bacteria 3
• The relationship between altered vaginal microflora and preterm birth is likely mediated by host immune responses 4
• Pathogenic bacteria may ascend from the lower reproductive tract into the uterus, with resulting inflammation triggering preterm labor and membrane rupture 5
• The inflammatory response includes elaboration of cytokines and proteolytic enzymes that can lead to cervical changes and membrane weakening 5

Risk Stratification

• BV prevalence in pregnancy ranges from 9-23% in academic and public hospital settings 1
• Higher prevalence is observed among:
  • African-American women 1
  • Women of low socioeconomic status 1
  • Women who previously delivered low-birthweight infants 1
• Women with prior preterm delivery are considered at particularly high risk when they have BV 1
• The prevalence of BV is significantly higher in women with threatened preterm labor and preterm labor compared to women in term labor 6

Screening and Treatment Considerations

• Current guidelines do not recommend routine screening for BV in asymptomatic pregnant women at low risk for preterm delivery 1
• For asymptomatic pregnant women at high risk for preterm delivery (particularly those with previous preterm birth), evidence regarding screening and treatment is conflicting 1
• Treatment is appropriate for pregnant women with symptomatic BV infection 1
• Treatment options include:
  • Metronidazole 500 mg orally twice daily for 7 days 1
  • Metronidazole gel 0.75%, one full applicator (5g) intravaginally, once daily for 5 days 1
  • Clindamycin cream 2%, one full applicator (5g) intravaginally at bedtime for 7 days 1

Important Caveats

• Despite the clear association between BV and preterm birth, treatment of asymptomatic BV during pregnancy has not consistently been shown to improve preterm birth rates 4
• Some studies suggest treatment may actually increase preterm birth in certain populations 1
• In studies showing benefit, screening was typically performed in the second trimester (13-24 weeks) 1
• Treatment benefit appears most significant in women with particularly high risk (35-57% baseline risk of preterm delivery) 1
• Up to 50% of BV cases may resolve spontaneously during pregnancy 1
• The optimal timing of screening and treatment regimen remains unclear 1

In conclusion, while BV clearly increases the risk of preterm labor and other adverse pregnancy outcomes, the approach to screening and treatment should be guided by individual risk factors, particularly history of previous preterm birth.