Beta-endorphin concentrations in fetal blood during the second half of pregnancy.

OBJECTIVE: Endogenous opiates may play a role in both fetal physiologic functions and the adaptation to intrauterine stress. However, our understanding of this role is hampered by an absence of data on circulating levels of these substances during fetal life. STUDY DESIGN: We measured serum beta-endorphin values with a radioimmunoassay in 81 paired fetal and maternal blood samples and 24 neonatal cord specimens. The former samples were uneventfully obtained from uncomplicated pregnancies between 18 and 39 weeks of gestation at the time of cordocentesis for prenatal diagnosis. RESULTS: Mean fetal beta-endorphin concentrations were significantly lower than beta-endorphin values from neonates (90.5 pg/ml [+/- 59.4] vs 228.4 pg/ml [+/- 166.2]; p less than 0.001), but significantly higher than mean maternal values (70.5 pg/ml [+/- 48.8]; p less than 0.02). Although fetal beta-endorphin levels decreased between 18 and 28 weeks' gestation, the correlation between fetal beta-endorphin values and gestational age was not significant (r = -0.193; p = 0.07). However, fetal beta-endorphin concentrations were significantly correlated with maternal values (Spearman's rank r = 0.47; p less than 0.001). CONCLUSION: These findings suggest that delivery or fetal adaptation to an extrauterine environment is associated with significant increases in beta-endorphin release. Moreover, although the fetal pituitary may be the primary source of circulating fetal beta-endorphin, a maternal or placental contribution cannot be excluded. Our data identify a physiologic range for fetal beta-endorphin concentrations.