The best way to determine whether probiotics can reduce the risk of PTB is by performing a type of study called a double-blind, randomised controlled trial (RCT). In such a trial, the participants are allocated to receive either probiotic supplements or dummy (placebo) supplements. The allocation to a particular supplement is purely by chance (random) and neither the participants nor the researchers know the allocation of any participant until the end of the trial (double-blind). Then, by looking at the difference in the rate of PTB between the groups it would be possible to say whether probiotics can reduce the rate of PTB. The investigators have estimated that an RCT that could detect a useful difference in the rate of PTB between the two groups would require the participation of approximately 10,000 women and cost several million pounds. Given that approximately 20,000 deliveries occur annually in inner North East London alone, there are sufficient women in this region who would be eligible for participation in such a RCT. However, the willingness of women to participate in and complete such a trial is unknown.