The interaction between vaginal microbiota, cervical length, and vaginal progesterone treatment for preterm birth risk

Abstract

Background: Preterm birth is the primary cause of infant death worldwide. A short cervix in the second trimester of pregnancy is a risk factor for preterm birth. In specific patient cohorts, vaginal progesterone reduces this risk. Using 16S rRNA gene sequencing, we undertook a prospective study in women at risk of preterm birth (n = 161) to assess (1) the relationship between vaginal microbiota and cervical length in the second trimester and preterm birth risk and (2) the impact of vaginal progesterone on vaginal bacterial communities in women with a short cervix.

Results: Lactobacillus iners dominance at 16 weeks of gestation was significantly associated with both a short cervix <25 mm (n = 15, P < 0.05) and preterm birth <34⁺⁰ weeks (n = 18; P < 0.01; 69% PPV). In contrast, Lactobacillus crispatus dominance was highly predictive of term birth (n = 127, 98% PPV). Cervical shortening and preterm birth were not associated with vaginal dysbiosis. A longitudinal characterization of vaginal microbiota (<18, 22, 28, and 34 weeks) was then undertaken in women receiving vaginal progesterone (400 mg/OD, n = 25) versus controls (n = 42). Progesterone did not alter vaginal bacterial community structure nor reduce L. iners-associated preterm birth (<34 weeks).

Conclusions: L. iners dominance of the vaginal microbiota at 16 weeks of gestation is a risk factor for preterm birth, whereas L. crispatus dominance is protective against preterm birth. Vaginal progesterone does not appear to impact the pregnancy vaginal microbiota. Patients and clinicians who may be concerned about "infection risk" associated with the use of a vaginal pessary during high-risk pregnancy can be reassured.

Keywords: Cervical length; Lactobacillus; Preterm birth; Progesterone; Vaginal microbiome.

Fig. 1

L. iners dominance is associated with a short cervix and preterm birth risk while L. crispatus is protective. a Heatmap of vaginal species data correlated community state types of samples (n = 161) with ethnicity, cervical length <25 mm, subsequent cerclage or progesterone intervention, and gestation at birth. b A short cervix <25 mm at 16 weeks was associated with a higher prevalence of L. iners (9/15, 60%) than longer cervical length (45/146, 31%, P = 0.04, two-tailed Fisher’s exact). c L. iners dominance was associated with early preterm birth <34⁺⁰ weeks (12/18, 67%), but not late preterm birth, 34⁺⁰ to 36⁺⁶ weeks (5/16, 31%) or term birth (37/127, 29%, P = 0.003). A greater proportion of term births had L. crispatus dominance at 16 weeks (63/127, 46%) than both late preterm (5/16, 31%) and early preterm births <34⁺⁰ weeks (2/18, 11%; P = 0.009; Fisher’s exact). d A Kaplan-Meier survival curve demonstrated that L. iners (n = 54) dominance at 16 weeks is associated with earlier gestation at delivery than a microbiome dominated by L. crispatus (n = 65, P = 0.02; Gehan-Breslow-Wilcoxon test)

Fig. 2

Vaginal progesterone treatment does not alter structure of the vaginal microbiome. a Compared to controls (n = 42), progesterone supplementation (n = 25) had no significant impact upon microbial community profiles with advancing gestation. Similarly, no effect of progesterone treatment upon b the number of species observed or c the corresponding Shannon index of alpha diversity was observed (2-way ANOVA). Fewer women requiring progesterone had a L. crispatus dominated microbiome compared to controls (8/25, 32 vs. 18/42, 43%, P = 0.4); however, progesterone treatment was associated with increased relative L. crispatus abundance with advancing gestation. Advancing gestational age from 18 to 34 weeks was not associated with a significant shift in mean relative abundance of L. iners (d) or L. crispatus (e) in either the controls or progesterone groups (Kruskal-Wallis, Dunn’s multiple comparison)

Fig. 3

Longitudinal profiling of community state types for progesterone (n = 25) and control groups (n = 42). Progesterone supplementation was commenced after the first sampling time point (<18 weeks). Each longitudinal sample was assigned to a CST (Fig. 1a) as indicated by the color-coded rectangle and categorized as a function of delivery gestation

Fig. 4

Preterm birth, despite vaginal progesterone, is associated with L. iners dominance throughout pregnancy. Longitudinal sampling of 25 women receiving progesterone for a short cervix showed L. iners dominance was associated with all women who subsequently delivered preterm <34⁺⁰ weeks (n = 4; (**P < 0.05; Fisher’s exact). Single asterisk indicates the delivery samples collected within 2 weeks of delivery between 28 and 34 weeks