Habitual bidet toilets are widely used these days for cleaning the anal or urogenital area in many women. Although there might be some concerns that the use of bidet toilets may alter vaginal flora and cause abnormal vaginal colonization, the actual association between habitual bidet toilet use and the change of vaginal normal flora and the risk of subsequent preterm birth is rarely investigated. Here, we aimed to evaluate the effect of bidet toilets on abnormal vaginal colonization and preterm birth in patients with preterm labor (PTL) and preterm premature rupture of membrane (PPROM). This is a prospective cohort study of 212 women who admitted to high-risk pregnancy unit due to PTL or PPROM in two tertiary hospitals between 2015 and 2017. Response to a questionnaire containing the use, mode, or the time of bidet toilet usage and vaginal culture were obtained. The bidet toilet user was defined as those who have used bidet toilet since at least one year before pregnancy and during pregnancy. Maternal baseline characteristics, vaginal culture results, obstetrical outcome and neonatal outcome were collected and compared between the user and non-users of bidet toilet. Abnormal vaginal colonization included aerobe, anaerobe, and Ureaplasma species. Among 212 subjects, 67 (32.0 %) women responded that they used bidet toilet. The incidence of preterm birth before 37 weeks of gestation was significantly higher in the users compared to the non-users (86.8% vs. 72.8%, P=0.047). Interestingly, the rate of late preterm birth (GA 34+0-36+6 weeks) in the users was more than twice than the non-users (43.4% vs. 20.6%, P=0.003). Abnormal vaginal colonization rate was significantly higher in the users than the non-users (60.3% vs. 42.7%, P=0.027). Notably, the Escherichia coli colonization rate was higher in users than non-users (11.9% vs. 3.2%, P=0.039). The colonization rates of other bacterial types or genital Ureaplasma were not associated with use of bidet. Our study results implicate that the use of bidet toilet is associated with a higher rate of preterm birth and abnormal vaginal colonization in women with PTL and PPROM. Interestingly, E. coli colonization rate was significantly higher in bidet users.