Aim Recent reports have shown lower levels of Clostridium and higher levels of Lactobacillales in the intestinal microbiota in preterm birth patients compared to term birth patients. However, the influence of probiotics on perinatal status has not been elucidated. The aim of our study was to evaluate the effects of probiotics on perinatal outcomes.

Methods We retrospectively evaluated the effects of oral probiotics on perinatal outcome in patients at high risk of preterm birth. Probiotics containing Streptococcus faecalis, Clostridium butyricum and Bacillus mesentericus were administered for prophylaxis of bacterial vaginosis or treatment of constipation starting at 12.5 ± 4.1 weeks until delivery. Patients not administered probiotics were defined as the non‐probiotics group. Between these two groups, perinatal outcomes including gestational age at birth, birth weight, chorioamnionitis or funisitis and preterm birth before 32 weeks were compared. In addition, multivariate regression analyses were performed to evaluate factors influencing preterm birth before 32 weeks, chorioamnionitis/funisitis and normal vaginal flora.

Results The probiotics group showed longer gestation, higher birth weight, lower rates of chorioamnionitis and higher rates of normal vaginal flora compared to the non‐probiotics group. Multivariate regression analysis showed that probiotics significantly suppressed preterm birth before 32 weeks and tended to suppress chorioamnionitis/funisitis. The adjusted odds ratios (95% confidence interval) for preterm birth before 32 weeks and chorioamnionitis/funisitis were 0.05 (0.01–0.71) and 0.07 (0.01–1.03), respectively.