| Summary: | Introduction: The majority of pregnant women with a short cervix will deliver at term and, thus, may unnecessarily receive advanced monitoring and
treatment. It is still necessary to define more accurately which sub-population of women with a short cervix is at elevated risk for early delivery. Objective:
To determine if vaginal microbiome composition influenced the rate of spontaneous preterm birth in women with a short cervical length. Methods: In an
exploratory, observational prospective study, vaginal secretions were obtained from 591 women at 21–24 week gestation. Vaginal microbiome composition
was determined by analyzing the V1–V3 region of the bacterial 16S ribosomal RNA gene. Results: Lactobacillus crispatus was numerically dominant in
the vagina in 41.7% of subjects, followed by L. iners in 32% and Gardnerella vaginalis in 12%. In women whose cervix was ≤25mm, the sensitivity to
predict an spontaneous preterm birth was 11.8%. However, when L. crispatus was not the dominant vaginal bacterium, this sensitivity increased to 81.8%.
Similarly, in women with a cervical length ≤30mm, the sensitivity to predict an spontaneous preterm birth increased from 21.7 to 78.3% when L. crispatus
was not the dominant vaginal bacterium. In women with a prior spontaneous preterm birth and a cervix ≤25 or ≤30mm, L. crispatus dominance was also
associated with a reduced rate of spontaneous preterm birth in the current pregnancy (p<0.001). Conclusion: In pregnant women with a cervix ≤25mm or
≤30mm, the risk for an spontaneous preterm birth is increased if L. crispatus is not dominant in the vagina.
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