Lack of analgesic effect of systemically administered morphine or pethidine on labour pain

Objective To evaluate the true analgesic effect of morphine and pethidine on labour pain. Design The analgesic and sedative effects of intravenous morphine or pethidine and their effect on anxiety were compared in a prospective, double-blind, randomised dose-response study. Setting A Stockholm teaching hospital obstetric unit. Participants Ten healthy nulliparous parturients in active labour were included in each group. Results Even after repeated doses (up to 0.15 mg/kg body weight morphine and up to 1.5 mg/kg body weight pethidine) the findings were uniform, with very high pain scores maintained in each group as assessed with visual analogue scale. The parturients were all significantly sedated and several fell asleep but were awakened by pain during contractions. Conclusion It is concluded that labour pain is not sensitive to systemically administered morphine or pethidine. These drugs only cause heavy sedation. It therefore seems unethical and medically incorrect to meet parturients' requests for pain relief by giving them sedation. Considering the well documented negative effects on newborn infants we also believe systemic pethidine should be avoided in labour.