Babies born preterm (before completing 37 weeks in the womb) are at increased risk of long-term disability and death. The investigators do not fully understand the cause(s) of preterm birth but it occurs more frequently when the normal, healthy bacteria (called Lactobacilli) in a woman's birth canal are replaced with unhealthy bacteria. Previous attempts to get rid of the unhealthy bacteria with antibiotics have not shown to affect the risk of preterm birth. The reason for this may be that what is required is the replacement of Lactobacilli in the birth canal. This can be done by asking women to take capsules containing lactobacilli once daily. To study whether oral Lactobacilli capsules compared with dummy capsules can reduce the risk of preterm birth, a large study involving approximately 10,000 women would be required. But the investigators do not know whether women would agree to take part in and complete such a study, and this is what the investigators wish to study in the small, initial study described here. The results of this study will show whether probiotics produce the desired biological effects on vaginal bacteria, and whether it would be feasible to perform the larger, definitive study of their effectiveness in prevention of preterm birth.