BACKGROUND: Preterm birth causes 60% to 80% of neonatal deaths. Survivors can experience life-long complications. The risk of preterm labour in the presence of maternal infection is thought to be 30% to 50%. Probiotics are defined as live micro-organisms which, when administered in an adequate amount, confer a health benefit on the host. They have been shown to displace and kill pathogens and modulate the immune response by interfering with the inflammatory cascade that leads to preterm labour and delivery. During pregnancy, local treatment restoring normal vaginal flora and acidity without systemic effects could be preferable to other treatment in preventing preterm labour. OBJECTIVES: To evaluate the effectiveness and the safety of probiotics for preventing preterm labour and birth. SEARCH STRATEGY: We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (June 2006). SELECTION CRITERIA: All randomised controlled trials assessing the prevention of preterm birth in pregnant women and women planning pregnancy through the use of probiotics to treat or prevent urogenital infections. DATA COLLECTION AND ANALYSIS: We extracted data using the prepared form and analysed with the Review Manager software. MAIN RESULTS: We assessed four trials for inclusion in the review. One trial started in February 2005 and is still ongoing. We excluded one trial because there were no data to be extracted from the article. Of the two trials included in the review, one enrolled women after 34 weeks of pregnancy using oral fermented milk as probiotic, while the other study utilised commercially available yogurt to be used vaginally by women diagnosed with bacterial vaginosis in early pregnancy. Reduction in genital infection was the only prespecified clinical outcome for which the data were available; pooled results showed an 81% reduction in the risk of genital infection with the use of probiotics (risk ratio 0.19; 95% confidence interval 0.08 to 0.48). AUTHORS' CONCLUSIONS: Although the use of probiotics appears to treat vaginal infections in pregnancy, there are currently insufficient data from trials to assess impact on preterm birth and its complications.