Background: Sterile water injections have been used as an effective intervention for the management of back pain during labour. The objective of the current research is to determine if sterile water injections, as an intervention for back pain in labour, will reduce the intrapartum caesarean section rate.Methods/design: Design: A double blind randomised placebo controlled trial. Setting: Maternity hospitals in Australia. Participants: 1866 women in labour, ≥18 years of age who have a singleton pregnancy with a fetus in a cephalic presentation at term (between 37 + 0 and 41 + 6 weeks gestation), who assess their back pain as equal to or greater than seven on a visual analogue scale when requesting analgesia and able to provide informed consent.Intervention: Participants will be randomised to receive either 0.1 to 0.3 millilitres of sterile water or a normal saline placebo via four intradermal injections into four anatomical points surrounding the Michaelis' rhomboid over the sacral area. Two injections will be administered over the posterior superior iliac spine (PSIS) and the remaining two at two centimetres posterior, and one centimetre medial to the PSIS respectively.Main outcome measure:Proportion of women who have a caesarean section in labour.Randomisation: Permuted blocks stratified by research site.Blinding (masking):Double-blind trial in which participants, clinicians and research staff blinded to group assignment.Funding:Funded by the National Health and Medical Research Council. Trial registration:Australian New Zealand Clinical Trials Registry (No ACTRN12611000221954).Discussion: Sterile water injections, which may have a positive effect on reducing the CS rate, have been shown to be a safe and simple analgesic suitable for most maternity settings. A procedure that could reduce intervention rates without adversely affecting safety for mother and baby would benefit Australian families and taxpayers and would reduce requirements for maternal operating theatre time. Results will have external validity, as the technique may be easily applied to maternity populations outside Australia. In summary, the results of this trial will contribute High level evidence on the impact of SWI on intrapartum CS rates and provide evidence of the analgesic effect of SWI on back pain.