Plasma beta-endorphin concentrations prior to and during pregnancy, in labor, and after delivery.

Beta-Endorphin was measured by radioimmunoassay in peripheral plasma of nonpregnant women (58 +/- 2.4 pg/ml, n = 17, mean +/- SE), during the first trimester (47 +/- 2.4 pg/ml, n = 11), the second trimester (33 +/- 1.9, n = 11), and the third trimester (49 +/- 2.7 pg/ml, n = 10) of pregnancy, during early (202 +/- 32 pg/ml, n = 12) and advanced labor (389 +/- 78 pg/ml, n = 10), and 30 to 60 minutes post partum (177 +/- 22 pg/ml, n = 12). Mean plasma levels of beta-endorphin were significantly lower in each trimester of gestation than the levels in nonpregnant control subjects. During labor and the early postpartum period, maternal plasma levels of beta-endorphin were significantly elevated. Furthermore, peripheral plasma levels of beta-endorphin during labor fell from 189 +/- 31 to 97.6 +/- 12 pg/ml (n = 13, p = 0.015) in response to epidural anesthesia, as compared to peripheral plasma concentrations of beta-endorphin of 223 +/- 71 and 193 +/- 47 pg/ml prior to and after injection of saline solution into epidural catheters, respectively, in 10 control subjects. Mean plasma levels of beta-endorphin in patients immediately prior to elective repeat cesarean section who were not in labor (151 +/- 23 pg/ml, n = 15) were significantly higher (p less than 0.005) than the levels in third-trimester control subjects. These data indicate that the pain associated with labor and the psychological stress of anticipating an operation are potent stimuli for the pituitary release of beta-endorphin.