BACKGROUND Intramuscular pethidine is routinely used throughout the UK for labour analgesia. Studies have suggested that pethidine provides little pain relief in labour and has a number of side effects affecting mother and neonate. It can cause nausea, vomiting and dysphoria in mothers and can cause reduced fetal heart rate variability and accelerations. Neonatal effects include respiratory depression and impaired feeding. There are few large studies comparing the relative side effects and efficacy of different opioids in labour. A small trial comparing intramuscular pethidine with diamorphine, showed diamorphine to have some benefits over pethidine when used for labour analgesia but the study did not investigate the adverse effects of either opioid. METHODS The Intramuscular Diamorphine versus Intramuscular Pethidine (IDvIP) trial is a randomised double-blind two centre controlled trial comparing intramuscular diamorphine and pethidine regarding their analgesic efficacy in labour and their side effects in mother, fetus and neonate. Information about the trial will be provided to women in the antenatal period or in early labour. Consent and recruitment to the trial will be obtained when the mother requests opioid analgesia. The sample size requirement is 406 women with data on primary outcomes. The maternal primary outcomes are pain relief during the first 3 hours after trial analgesia and specifically pain relief after 60 minutes. The neonatal primary outcomes are need for resuscitation and Apgar Score <7 at 1 minute. The secondary outcomes are an additional measure of pain relief, maternal sedation, nausea and vomiting, maternal oxygen saturation, satisfaction with analgesia, whether method of analgesia would be used again, use of Entonox, umbilical arterial and venous pH, fetal heart rate, meconium staining, time from delivery to first breath, Apgar scores at 5 mins, naloxone requirement, transfer to neonatal intensive care unit, neonatal haemoglobin oxygen saturation at 30, 60, 90, and 120 mins after delivery, and neonatal sedation and feeding behaviour during first 2 hours. DISCUSSION If the trial demonstrates that diamorphine provides better analgesia with fewer side effects in mother and neonate this could lead to a change in national practice and result in diamorphine becoming the preferred intramuscular opioid for analgesia in labour. TRIAL REGISTRATION ISRCTN14898678Eudra No: 2006-003250-18, REC Reference No: 06/Q1702/95, MHRA Authorisation No: 1443/0001/001-0001, NIHR UKCRN reference 6895, RfPB grant PB-PG-0407-13170_IR5.