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. Β -Lipotropin (ΒLPH) and Β-endorphin (ΒEP) 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 (ΒLPH: 296.0 ± 60.2 pg/ml; ΒEP: 106.2 ± 40.6 pg/ml) as compared with values of 157.0 ± 35.4 (ΒLPH) and 57.0 ± 7.3 pg/ml (ΒEP) (mean ± SE) at the beginning of labour. With the exception of 1 case, a significant correlation was observed between ΒLPH and Β ΒEP plasma levels. The two opioid plasma levels decrease after delivery (195.3 ± 47.1 and 88.3 ± 39.7 pg/ml, respectively), but increase again 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 (ΒLPH) and 33.7 ± 16.5 (ΒEP) on the 5th day of puerperium. ΒLPH and ΒEP 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.