Cervical Microbiome Diversity May Contribute to High Preterm Birth Rates in Black Women

Black women have a greater degree of cervical microbiome diversity during pregnancy than white women, according to a new study. The findings reveal an important potential link between the cervical microbiome and the risk for preterm birth.

Sarahn Wheeler, MD, of the Division of Maternal Fetal Medicine in the Department of Obstetrics and Gynecology at Duke University, presented results from the prospective study at the 37th Annual Pregnancy Meeting of the Society for Maternal Fetal Medicine held January 23 to 28, 2017, in Las Vegas, NV.

Black women experience disproportionately high rates of preterm birth (delivery < 37 weeks of gestation) compared with other racial/ethnic groups, even after controlling for socioeconomic status and other risk factors. However, the causes underlying racial disparities in preterm birth rates remain unclear.

“We know that there is a greater diversity of species comprising the natural vaginal microbiome in black women than in white women,” Wheeler said. She hypothesized that vaginal organisms may be traveling through the cervix and into the amniotic cavity, where they are infecting the fetus and causing preterm birth.

In the study, Wheeler and colleagues examined cervical microbiome diversity as a potential contributing factor for increased preterm birth rates among black women. The researchers enrolled 51 pregnant women in their first or second trimester, including 39 non-Hispanic black women (76%) and 12 non-Hispanic white women (24%).

All study participants underwent cervix sampling with a sterile swab for microbiome analysis and cervicovaginal lavage for inflammatory marker assessment. The degree of cervical microbiome diversity was scored using the Shannon Diversity Index (SDI), which is a standardized scoring system for bacterial DNA diversity.

Baseline characteristics were similar in both groups, with no differences observed in maternal age, body mass index, or gestational age at the time of specimen collection (Table). The rates of pregnancy complications such as preeclampsia and comorbidities, including hypertension, diabetes, and tobacco use, were also similar in both groups.

Table. Maternal Characteristics and Pregnancy Outcomes

Characteristic Non-Hispanic Blacks
(n = 39)
Non-Hispanic Whites
(n = 12)
P Value
Mean age at delivery, y 30.4 30.6 .74
Mean body mass index, kg/m² 33.7 38.5 .2
Mean gestational age at cervical microbiome collection, wk 15.1 16.7 .3
Median SDI score (interquartile range) 0.6 (0.2-1.3) 0.13 (0.1-0.6) .03
Preterm birth rate, % 35.9 0 .02
Mean gestational age at delivery, wk 37.0 39.2 < .001
Mean birth weight, g 2971.9 3577.2 < .001

SDI = Shannon Diversity Index.

Black women had significantly higher median SDI scores, indicating greater cervical microbiome diversity compared with white women (P = .03). According to Wheeler, this is the first study to demonstrate racial differences in cervical microbiome diversity among pregnant women.

Several pregnancy outcomes also differed between groups. Among 14 preterm deliveries that occurred in the study cohort, all occurred in black women, resulting in a preterm delivery rate of 35.9% for black women and 0% for white women (P = .02). In addition, the mean birth weight was significantly lower among black women than among white women (2971.9 vs 3577.2 g; P < .001).

There was a trend toward higher median SDI scores in women with preterm births compared with term deliveries, although this difference was not statistically significant (0.9 vs 0.4; P = .06).

To examine the possible role of inflammation as a mediator of preterm birth, the research team also compared levels of multiple cervical and vaginal inflammatory markers by race (black vs white), delivery timing (term vs preterm), or cervical microbiome diversity. In general, no significant differences were observed across subgroups in the median levels of interleukin (IL) 1α, IL-1β, IL-6, IL-8, interferon-inducible protein 10, monocyte chemoattractant protein 1, and tumor necrosis factor in the cervicovaginal lavage samples. However, a weak correlation was observed between IL-1β and cervical microbiome diversity (P = .05).

“These findings suggest that, if there is a relationship between cervical microbiome diversity and preterm birth, then it may be mediated by a mechanism other than inflammation,” Wheeler explained.

Ultimately, this line of research may lead to better tools for preterm birth risk profiling and possibly clinical interventions. “If we can get down to the individual species level and identify which bugs are the culprits in preterm birth, then we may be able to develop therapeutics,” said Wheeler.

Source: Wheeler SM, Pryor K, Antczak B, et al. Race and cervical microbiome diversity. Presented at: Society for Maternal-Fetal Medicine 37th Annual Pregnancy Meeting; January 23-28, 2017; Las Vegas, NV. Abstract 291.