Background Spontaneous preterm birth (SPTB) is a major cause of neonatal morbidity and mortality worldwide and defining its risk factors is necessary to reduce its prevalence. Recent studies have pointed out that bacterial vaginosis, a disturbance in the vaginal microbiome, is associated with SPTB. It is hypothesized that vaginal hygiene practices can alter the vaginal microbiome and are therefore associated with SPTB, but there are no studies investigating this matter. Methods and findings A case-control study was conducted between August 2018 and July 2021 in two affiliated university medical centers in Amsterdam, the Netherlands. We included a total of 79 women with a SPTB and compared them with 156 women with a term birth. Women with uterine anomalies, a history of cervical surgery or major congenital anomalies of the fetus were excluded. All participants filled in a questionnaire about vaginal washing with water, soap or gel, the use of intravaginal douches and vaginal steaming, both before and during pregnancy. Most women washed vaginally with water, 144 (61.3%) women before pregnancy and 135 (57.4%) women during pregnancy. A total of 43 (18.3%) washed with soap before and 36 (15.3%) during pregnancy. Before pregnancy, 40 (17.0%) women washed with vaginal gel and 27 (11.5%) during pregnancy. We found that the use of vaginal gel before pregnancy (aOR 2.29, 95% CI: 1.08–4.84) and even more during pregnancy, was associated with SPTB (aOR 3.45, 95% CI: 1.37–8.67). No association was found between washing with water or soap, intravaginal douching, or vaginal steaming and SPTB. Conclusions Our findings suggest that the use of vaginal gel is associated with SPTB. Women should be informed that vaginal use of gels might not be safe.

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