The relationship between bacterial vaginosis (BV) and preterm delivery has become well known in recent years, although there are few studies on: (i) the differences in test results during the early gestational (EGP) and middle gestational (MGP) periods; (ii) the significance of the intermediate (I) group that does not develop overt BV; or (iii) the therapeutic effects of metronidazole. We performed a retrospective study to analyze the relationship between the vaginal bacterial status and the preterm delivery rate. Without treatment, the preterm delivery rate was higher in the BV subgroup than in the I and normal (N) subgroups (p = 0.021) in the EGP, whereas the rates in the BV and I subgroups were higher than in the N subgroup in the MGP (p = 0.0003). Although treatment of BV by metronidazole vaginal tablets significantly increased the N subgroup in the MGP (p = 0.020), there was no significant improvement in the preterm delivery rate. Decreasing the rate of preterm delivery requires development of treatment methods that will further increase the percentage of patients who test N during the MGP after BV during the EGP.