β-Endorphin during spontaneous and oxytocin-induced labour

$-ENDORPHIN concentration in maternal blood has been shown to increase significantly during labour until delivery (Facchinetti et ul., 1983; Fettes et ul., 1984). It has been suggested that this increase is of importance for the analgesic and behavioural properties of the peptide (Krieger and Martin, 1981) and could be a response to physical stress or may be specifically related to pain perception (Fletcher ef ul., 1980; Raisanen ef ul., 1984). The clinical observation that the oxytocininduced labour is often associated with marked tocographic reactions and less tolerable pain suggests the possible impairment of the antinociceptive system during oxytocin infusion. Recently, Genazzani ef ul. (1985) reported that $-endorphin and $-lipotropin concentration in maternal blood remained constant during the infusion of oxytocin to stimulate uterine contractions during labour. However, their study did not clarify whether $-endorphin concentration would rise or not in oxytocin-induced labour compared to spontaneous labour. This question is the basis of the present investigation. PATIENTS AND METHODS We studied $-endorphin concentration in the blood and amniotic fluid of 26 healthy pregnant women at term. In 11 women labour started spontaneously and a 0.9 per cent saline infusion (500ml) was given; in the other 15, selected at random, labour was induced with oxytocin, 5i.u. in 500ml of 0.9 per cent saline, mean infusion rate 16 dropdmin, 8m.d min; the infusion was maintained until delivery. Maternal blood was collected from the antecubital vein in EDTA containing tubes before labour, at 5 cm cervical dilatation and at delivery. Amniotic fluid was collected by artificial rupture of the membranes during amnioscopy at 5 cm dilatation and at delivery; only fluids not contaminated with blood or