Gestational diabetes mellitus (GDM) is a common complication of pregnancy, and previous studies have shown that pregnant women with GDM have a higher risk of developing infectious diseases of the reproductive tract than healthy pregnant women, but relevant cohort studies are rare.

To investigate the variation characteristics of vaginal microbiota in pregnant women with GDM at different gestational weeks, to compare the differences of vaginal microbiota between GDM and non-GDM women, and to observe the effects of vaginal microbiota changes on pregnancy outcomes.

In this study, we used a consecutive sampling method to recruit pregnant women who established their records and regulated health care at Daxing Teaching Hospital of Capital Medical University from March 2022 to March 2023. They underwent 75 g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation to diagnose GDM. The GDM group was matched 1∶1 with the non-GDM group using propensity score matching method (PSM). Sample of vaginal secretions were collected at 24-28 weeks (stage A), 32-35 weeks (stage B) and 37-40 weeks (stage C) for three vaginal microecological examination, respectively. Based on the results, participants were further divided into the GDM normal vaginal flora subgroup, GDM abnormal vaginal flora subgroup, non-GDM normal vaginal flora subgroup and non-GDM abnormal vaginal flora subgroup, and were followed up until 7 d after delivery to assess pregnancy outcome.

A total of 426 participants were initially enrolled in this study, and GDM occurred in 141 cases. After PSM mathing, the GDM and non-GDM groups were successfully matched in 122 pairs. In this study, because 9 women with GDM and 5 women without GDM had preterm birth (<37 weeks of gestation), 113 women with GDM and 117 women without GDM ended up in stage C. The proportion of the dominant vaginal flora of Lactobacillus was higher in stage A than in stage C. Moreover, the vaginal pH, the incidence of vaginal flora abnormalities, and the incidence of BV and VVC were lower than those in stage C (P<0.05). The proportion of dominant bacteria as Lactobacillus in the GDM group was higher than that in the non-GDM group, and the incidence of abnormal vaginal flora and VVC was lower than that in the non-GDM group in stage A (P<0.05). In contrast, the proportion of the dominant bacterium Lactobacillus was lower in the GDM group than in the non-GDM group, and the incidence of abnormal vaginal flora was higher than in the non-GDM in stage C (P<0.05). The incidence of adverse pregnancy outcomes was higher in the GDM abnormal vaginal flora subgroup (n=65) than in the GDM normal vaginal flora subgroup (n=57) (P<0.05). In more details, the risk of adverse pregnancy outcomes in the GDM abnormal vaginal flora group was 1.830 times higher than that in the GDM normal vaginal flora group (RR=1.830, 95%CI=1.293-2.590, P<0.001) .

Compared with non-GDM pregnant women, GDM pregnant women had a lower incidence of vaginal flora abnormalities at 24-28 weeks of gestation and an increased risk of vaginal flora abnormalities after 37 weeks of gestation. GDM pregnant women with abnormal flora have higher risk of adverse pregnancy outcomes, so we recommend enhanced testing and management of vaginal microecology during pregnancy.