Endogenous opioid peptides in pregnancy.

Plasma levels of two endogenous opioid peptides, beta-endorphin and met-enkephalin, as well as immunoreactive N-terminal beta-lipotrophin (N-LPH) were studied in normal pregnant women from 8 to 41 weeks gestation. A total of 116 samples were assayed for beta-endorphin-like immunoreactivity (C-LPH), 103 for N-LPH and 75 for met-enkephalin. Plasma beta-endorphin-like immunoreactivity and N-LPH levels rose progressively throughout gestation and reached a maximum at term. Plasma met-enkephalin immunoreactivity did not significantly change throughout pregnancy. These results reflect an increasing and consistent adrenocorticotrophin (ACTH)/beta-LPH and beta-endorphin secretion throughout pregnancy, while the unchanged met-enkephalin levels are compatible with its known derivation from a separate precursor system.