Association of bacterial vaginosis and low birthweight infants [Classics Series]

1. Bacterial vaginosis (BV) is associated with 40% increased odds of preterm delivery of a low birthweight (LBW) infant.

Original Date of Publication: December 1995

Study Rundown: Preterm delivery, delivery before 37 weeks gestation and low birthweight, birthweight of <2,500 grams, infants are two of the most complicated problems in obstetrics. While the incidence of LBW infants declined from the late 1980’s to the early 1990’s, the incidence of premature low birthweight infants increased. This prompted investigation of risk factors for preterm delivery of low birthweight infants. Prior preterm delivery, low socioeconomic status and black race were previously identified risk factors for preterm delivery but no modifiable risk factors had been identified. A previous multicenter study of a large cohort of pregnant women identified an association of gential microorganisms with preterm delivery of a LBW infant. Bacterial vaginosis is an imbalance in the vaginal bacterial flora in which anaerobic bacteria (e.g. Gardnerella vaginalis, mycoplasma hominis, bacterodies) replace the lactobacillus-predominant vaginal flora. Theoretically, overgrowth of the genital tract with bacterial vaginosis might create an environment of local inflammation and cytokine production that might initiate a cascade of events leading to preterm parturition. The mechanism by which bacterial vaginosis might affect low birth weight likely occurs by virtue of precipitating delivery when the estimate fetal weight is <2,500 grams. In the present work, researchers assessed the incidences of bacterial vaginosis in the second trimester and preterm delivery of a low birthweight infant.

This landmark study demonstrated that the presence of bacterial vaginosis as diagnosed by vaginal pH and gram stain was associated with increased odds of preterm delivery of a LBW infant. Strengths included strict diagnostic criteria and blinding of patients and providers to bacterial vaginosis diagnosis, which decreased risk of iatrogenic preterm delivery. Limitations include lack of accounting for black race, cigarette smoking or previous LBW infant such that confounding by these factors cannot be ruled out and is likely. Following the results of this landmark trial, subsequent investigations demonstrated that the association of BV with PTD was unlikely to be causal and was most probably related to an increased risk of chorioamnionitis. Routine screening and treatment of asymptomatic pregnant women with BV has not been shown to reduce the rate of preterm delivery and is therefore not recommended.

Click to read the study in NEJM

Dr. Alan Peaceman, MD, talks to 2 Minute Medicine: Northwestern University School of Medicine; Chief, Division of Obstetrics and Gynecology-Maternal Fetal Medicine.

“This prospective cohort study identified an increased odds of preterm delivery of a low birthweight infant in women with bacterial vaginosis (BV). Subsequent investigations failed to demonstrate that treatment of asymptomatic BV in pregnant women, even those at increased risk for preterm delivery, reduced the incidence of preterm birth. As such, universal screening and treatment of BV in pregnancy is not recommended.”

In-Depth [prospective cohort study]: A total of 10 397 pregnant women with no risk factors for preterm delivery were assessed across seven medical centers for bacterial vaginosis from 23-26 weeks gestation and the incidence of preterm delivery of a LBW infant was assessed. Bacterial vaginosis was diagnosed on the basis of vaginal pH testing and gram stain. The primary outcome was preterm delivery prior to 37 weeks gestation of an infant below a birthweight of 2500 grams.

Bacterial vaginosis was associated with 40% increased odds of preterm delivery of a LBW infant (95% CI: 1.1-1.8). Among women with bacterial vaginosis, those with both vaginal bacteroides and mycoplasma hominis experienced the highest odds of preterm delivery of LBW infant (OR: 2.1, 95%CI1.5-3.0). Other significant risk factors for preterm delivery of LBW infant were history of prior LBW infant, smoking and black race; these risk factors were not included in the multivariate model used to generate effect estimates.

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