Influence of medication, pain and progress in labour on plasma beta-endorphin-like immunoreactivity.

Plasma beta-endorphin-like immunoreactivity (beta-ELI) was measured at intervals during labour in 42 patients. An increase in beta-ELI occurred during labour and increased to a maximum during the second stage. The increase in beta-ELI paralleled dilatation of the cervix. A correlation existed between the duration of labour and second stage beta-ELI in patients with labours of more than 4 h duration. Analgesic drugs were associated with changes in beta-ELI. The patients who received Entonox had greater plasma beta-ELI concentrations than those who received pethidine. A significant difference in the second stage beta-ELI existed between those who received only Entonox and those who received only pethidine. Induction and augmentation of labour were also associated with differences in beta-ELI. Patients in spontaneous labour had significantly lower beta-ELI than patients who had either artificial rupture of membranes or Syntocinon augmentation of labour.