Opioid plasma levels during labour.

6 healthy pregnant women, monitored with external cardiotocography, were studied. Blood samples were collected hourly throughout labour until delivery of the fetus and placenta, and again on the 5th day of puerperium. beta-Lipotropin (beta LPH) and beta-endorphin (beta EP) were determined by specific radioimmunoassays in each sample, after silicic acid plasma extraction and G-75 column chromatography. Both opioid plasma levels rise progressively during labour, reaching their highest values 1 h before delivery (beta LPH: 296.0 +/- 60.2 pg/ml; beta EP 106.2 +/- 40.6 pg/ml) as compared with values of 157.0 +/- 35.4 (beta LPH) and 57.0 +/- 7.3 pg/ml (beta EP) (mean +/- SE) at the beginning of labour. With the exception of 1 case, a significant correlation was observed between beta LPH and beta EP plasma levels. The two opioid plasma levels decrease after delivery of the placenta (272.5 +/- 59.6 and 118.2 +/- 68.4 pg/ml) and subsequently decrease further to levels of 105.6 +/- 46.4 (beta LPH) and 33.7 +/- 16.5 (beta RP) on the 5th day of puerperium. beta LPH and beta EP plasma levels showed a significant correlation throughout labour with the 'uterine contractility force/hour', which was calculated by the addition of tocographic areas of each uterine contraction at 1-hour intervals. These data demonstrate that the increase in plasma opioid concentrations during labor is directly related to the number and intensity of uterine contractions.