The bacterial community in the female lower genital tract has a vital role in maternal and neonatal health. Imbalances in this microbiota seem to be associated with negative reproductive outcomes, such as spontaneous preterm birth (sPTB). However, the mechanisms underlying the association between a disturbed microbiota and sPTB are not clearly known. A study was carried out to specify the vaginal microbiota of 46 pregnant women who had sPTB and compare it to that of pregnant women who delivered at term (n = 170). Vaginal swabs were collected at 11-16 weeks of gestational age. Microbiota profiles were created by PCR amplification and pyrosequencing of the cpn60 universal target region. Profiles clustered into seven community state types: I (Lactobacillus crispatus dominated), II (Lactobacillus gasseri dominated), III (Lactobacillus iners dominated), IVA (Gardnerella vaginalis subgroup B or mix of species), IVC (G. vaginalis subgroup A dominated), IVD (G. vaginalis subgroup C dominated) and V (Lactobacillus jensenii dominated). Women who experienced preterm birth (< 37 weeks gestation) had microbiota with higher richness and diversity and higher prevalence of Mollicutes when compared to those of women who delivered at term. The two groups did not cluster according to CST. Therefore, the researchers could not identify a specific microbial community structure that would predict sPTB, but differences in microbiota richness, diversity and Mollicutes prevalence were observed between groups. The findings of this study affirmed an association between Mollicutes and sPTB and suggested that a more diverse microbiome may play a role in the pathogenesis of some cases. The findings are published in Microbiome.