Bacterial Vaginosis (BV), a common vaginal infection, is strongly associated with an increased risk of delivering a baby prematurely. BV is considered a modifiable risk factor for Preterm Labor (PTL), defined as birth before 37 weeks of gestation. The presence of this bacterial imbalance in the lower genital tract can trigger a chain of events that influences the environment of the uterus and cervix. Understanding the biological pathway linking BV to early delivery is essential for developing effective prevention and management strategies during pregnancy. This relationship highlights why medical providers focus on identifying and treating infections, especially in women at elevated risk.
Defining Bacterial Vaginosis and Preterm Labor
Bacterial Vaginosis is the most frequent vaginal condition affecting women of reproductive age, characterized by a disruption of the natural vaginal flora. The healthy vaginal environment is normally dominated by beneficial Lactobacillus species, but BV occurs when these bacteria are significantly reduced and replaced by an overgrowth of various anaerobic organisms. Globally, the prevalence of BV in women of reproductive age is often estimated between 23 and 29%. While up to 84% of affected women may experience no symptoms, others may notice a thin, gray or white vaginal discharge often accompanied by a distinct, “fishy” odor.
Preterm Labor is defined by the onset of regular uterine contractions causing cervical changes before the 37th week of pregnancy. A premature birth is the leading cause of death and long-term illness in newborns globally. Infants born too early face immediate health challenges, including respiratory distress syndrome and necrotizing enterocolitis. Longer-term complications can involve neurodevelopmental impairments, such as cerebral palsy, vision problems, and hearing loss.
How BV Increases Risk of Premature Birth
The mechanism linking Bacterial Vaginosis to an increased risk of premature birth is largely understood as an ascending infection that triggers an inflammatory response. The excessive anaerobic bacteria associated with BV, such as Gardnerella vaginalis, Prevotella, and Mobiluncus, can migrate upward from the vagina into the upper reproductive tract. This ascent can lead to the colonization of the cervix and the membranes surrounding the fetus, creating a localized infection.
The presence of these pathogens and the byproducts they release causes a localized immune response within the uterus and surrounding tissues. Specifically, the body releases inflammatory signaling molecules, including cytokines and prostaglandins, to fight the infection. Prostaglandins are known to have a direct effect on the uterus, acting as potent stimulators of muscle contraction. This biological cascade can initiate labor, leading to preterm delivery, or cause the premature rupture of membranes (PROM).
The risk of preterm birth is significantly heightened in pregnant women with BV. The odds of preterm delivery are reported to be approximately two to three times higher in women with BV compared to those without the infection. This risk is particularly elevated when the infection is diagnosed early in the pregnancy, before 16 to 20 weeks of gestation.
Diagnosis and Management During Pregnancy
Diagnosis and management of Bacterial Vaginosis are important aspects of prenatal care due to the link with adverse pregnancy outcomes. Routine, universal screening for asymptomatic BV in all pregnant women is not currently recommended by major health organizations. Instead, screening is typically focused on high-risk individuals, primarily those with a previous history of spontaneous preterm birth.
Diagnosis Methods
Diagnosis relies on clinical criteria or laboratory analysis. The Amsel criteria require the presence of at least three of four signs:
- Thin discharge.
- A vaginal pH greater than 4.5.
- The presence of “clue cells” on microscopy.
- A positive “whiff test” for a fishy odor.
The more definitive laboratory method involves using the Nugent scoring system, which evaluates the vaginal flora composition on a Gram stain to determine the degree of bacterial imbalance.
Treatment is advised for all pregnant women who are symptomatic, or for those at high risk with a history of preterm birth, to potentially reduce the risk of premature rupture of membranes or preterm labor. The standard approach involves systemic antibiotic therapy, often using oral metronidazole or oral clindamycin. While topical antibiotics are effective for non-pregnant women, systemic treatment is preferred in pregnancy because it is thought to better reach the upper genital tract to address an ascending infection. Treating asymptomatic low-risk women with BV has not been shown to consistently reduce preterm birth rates, which is why management is targeted toward those with symptoms or known risk factors.