Gardnerella enrichment in the vaginal microbiome of women with gestational diabetes mellitus is associated with lower fetal birthweight percentiles

Aims/hypothesis We aimed to characterise alterations in the late-pregnancy vaginal microbiota in women with gestational diabetes mellitus (GDM), and to examine their associations with maternal glycaemic status and fetal growth. Methods In this observational case–control study, women with newly diagnosed, diet-managed GDM ( n =60) and healthy pregnant control participants ( n =119) were recruited at a single tertiary centre. Vaginal swabs were collected in the third trimester and analysed by full-length 16S rRNA gene sequencing. Associations between vaginal microbiota composition, maternal glycaemic measures, and fetal birthweight percentile (FBW%ile) were evaluated. Results The overall vaginal microbial composition differed between groups despite similar alpha diversity. Beta diversity analyses showed significant separation between the women with GDM and the control group ( R =0.068, p =0.011 by ANOSIM). Lactobacillus- dominated communities were less frequent in women with GDM than in control participants (73.3% vs 86.6%, p =0.029), with reduced Lactobacillus abundance (79.8% vs 87.4%, nominal p =0.020) and increased Gardnerella abundance (9.7% vs 1.4%, nominal p =0.011). Within the GDM group, Gardnerella abundance correlated positively with fasting glucose levels ( r =0.3617, nominal p =0.007) and inversely with FBW%ile ( r =−0.2774, nominal p =0.032). Stratification by FBW%ile (<50% vs ≥50%) showed a higher proportion of non- Lactobacillus- dominated communities (44.4% vs 12.1%, p =0.001) and higher Gardnerella abundance (15.0% vs 3.2%, nominal p =0.020) in the low FBW%ile subgroup. Associations with Gardnerella were attenuated in sensitivity analyses that excluded samples with extreme Gardnerella abundance. Conclusions/interpretation Late-pregnancy GDM was associated with modest shifts in vaginal microbial structure, characterised by reduced Lactobacillus dominance and relative enrichment of Gardnerella . Exploratory analyses of associations of Gardnerella with maternal fasting glucose levels and fetal growth suggest that variation in the vaginal microbial environment may contribute to metabolic and fetal growth heterogeneity in women with GDM.