In 2005, the World Health Organization estimated that 9.6% or 12.9 million births worldwide were born preterm at <37 weeks of gestation and were accompanied by a mortality rate as high as 42% (http://www.who.int/bulletin/volumes/88/1/08-062554). Significant data suggesting that intrauterine infection is an important modifier for the risk of preterm birth have emerged over the past four decades. However, causative microbial culprits have yet to be identified, and interventional trials with antimicrobials have uniformly failed to demonstrate a significant benefit. To the contrary, treatment for clinically asymptomatic, commonly associated polymicrobial communities (i.e., bacterial vaginosis) has resulted in an increase in the rate of preterm birth. This article discusses the importance of vaginal microbiome and the variance in its composition during normal pregnancy. We will expand this discussion to include possible mechanisms that might trigger preterm birth in at-risk subjects. Finally, we will review why preterm birth may be an ideal forum with which to apply our rapidly expanding metagenomic sequencing and analytic pipelines to discern the role of host and microbe in the relative continuum of health and disease.
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