A comparison of intramuscular diamorphine and intramuscular pethidine for labour analgesia: a two‐centre randomised blinded controlled trial

Intramuscular (i.m.) pethidine is used worldwide for labour analgesia and i.m. diamorphine usage has increased in the UK in the last 15 years. This trial aims to ascertain the relative efficacy and adverse effects of diamorphine and pethidine for labour pain.Prospective, parallel-arm randomised controlled trial with blinding of participants, care-givers and outcome assessors.Maternity units in two District General Hospitals in the UK.After written informed consent, 484 women were randomised and recruited (244 diamorphine, 240 pethidine). Inclusion criteria included women 16 years or older, established labour, singleton pregnancy, 37-42 weeks of gestation and weight 60-120 kg.On request of i.m. analgesia, participants received either 150 mg pethidine or 7.5 mg diamorphine based on computer-generated block randomisation.Maternal-reduction in pain intensity from baseline (10-cm visual analogue scale) at 60 minutes and over the 3-hour period after drug administration. Neonatal-requirement for resuscitation and Apgar score at 1 minute.Diamorphine provided modestly improved pain relief at 60 minutes, mean difference 1 cm (95% confidence interval [CI] 0.5-1.5), and over the 3 hours, mean difference 0.7 cm (95% CI 0.3-1.1). However, average length of labour in women receiving diamorphine was 82 minutes longer (95% CI 39-124) and therefore they experienced more pain overall. There were no statistically significant differences in primary neonatal outcomes.There is a modest difference between the analgesia provided by diamorphine or pethidine for labour analgesia but diamorphine is associated with significantly longer labours.