LAS VEGAS (FRONTLINE MEDICAL NEWS) – Certain cervicovaginal microbiota predispose women to spontaneous preterm birth, according to a new study, while other microbiota were found to be protective against preterm delivery.

The findings stand in contrast to previous clinical trials that “targeted uterine activity and/or uterine infection,” said Michal Elovitz, MD, the first author of a study presented at the annual Pregnancy Meeting sponsored by the Society for Maternal-Fetal Medicine.

Previous clinical trials that targeted myometrial activity saw no association with preterm birth rates, according to Dr. Elovitz. “If this paradigm is in error, then what are we missing about the pathogenesis of spontaneous preterm birth?” she asked.

To answer the question, Dr. Elovitz and her colleagues conducted a large prospective cohort and validation study, testing the hypothesis that cervicovaginal microbial communities in women “destined to have a preterm birth” would differ from those of control patients.

The Motherhood and the Microbiome study enrolled 1,500 women aged 13-50 years with singleton pregnancies to constitute the prospective cohort. Cervicovaginal swabs were obtained at three time points: at gestational weeks 16-20, 20-24, and 24-28. Dr. Elovitz , professor of ob.gyn. at the University of Pennsylvania, Philadelphia, was the study adjudicator for determining preterm births.

Within the prospective cohort, Dr. Elovitz and her collaborators identified 83 cases of spontaneous preterm birth (SPTB), and frequency matched them by race to 336 patients who had term deliveries in a 4:1 ratio. The cervicovaginal microbiota of these patients was characterized by performing 165 rRNA gene analyses. Once the bacterial composition and structure had been identified, the investigators then calculated the effect of cervicovaginal bacterial composition on the risk of SPTB by using the log ratio between the mean relative abundance of a given phylotype for the SPTB, compared with the term birth samples.

A second prospective cohort of 616 women was formed for validation; cervicovaginal specimens in this cohort were collected between 22 and 32 weeks of gestation.

Taking both cohorts together, a total of 127 phylotypes were found in all samples. A total of 13 bacterial species were positively associated with an increased risk of spontaneous preterm birth in the primary cohort; 37 species were significantly associated with a decreased risk of spontaneous preterm birth.

“Bifidobacterium species were noted to be significantly protective against SPTB at all gestational time points,” wrote Dr. Elovitz and her collaborators in the abstract accompanying the presentation. On the other hand, they wrote, “BVAB2, BVAB3, and Mobiluncus were associated with a dramatic increase[d] risk of SPTB (all q-values less than 0.0001).”

Abundant Mobiluncus mulieris, in particular, was associated with significantly increased risk of SPTB at all time points during the study. The odds ratio for SPTB with a positive mobiluncus swab at visit one was 9.21.

Since the study examined both relative and absolute abundance of individual bacterial species, the investigators were able to determine that when Bifidobacterium breve was present, the incidence of BVAB3-associated preterm birth dropped from more than 30% to less than 10% (P = .006).

The notion that specific cervicovaginal bacterial species can be associated with increased rate of SPTB represents a different finding than most previous work in this field, said Dr. Elovitz, adding that much of the research on the vaginal microbiome during the reproductive years has focused on groupings of predominant bacteria, termed community state types (CSTs). “Classification of communities into CST, as performed in the nonpregnant woman, is not revealing for PTB,” Dr. Elovitz said.

Future research into cervicovaginal microbial communities and spontaneous preterm birth are likely to produce new methods to risk-stratify women and potential new therapeutics to reduce the rate of spontaneous preterm birth, she added.

The presentation won the conference’s March of Dimes award for best abstract in prematurity.

Dr. Elovitz reported having no relevant financial disclosures.

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