To investigate the dynamic relationships among corticotropin-releasing hormone (CRH), beta-endorphin (beta EP), cortisol and obstetric events during pregnancy, blood samples were collected from 193 women at 28 weeks, 38 weeks, during labour and on the second postnatal day. Cord blood at delivery was also obtained. We found that: (1) Maternal plasma CRH, beta EP and cortisol rose from 28 to 38 weeks. (2) During the third trimester maternal plasma CRH and beta EP were correlated (r = 0.30, p < 0.001). (3) During labour, no correlations were found among maternal plasma CRH, beta EP and cortisol. (4) Maternal CRH at labour and the duration of labour were not correlated. (5) Maternal plasma CRH tended to be higher in women who delivered early (more than seven days prior to estimated date of confinement [EDC]) relative to those who were on time (within seven days' EDC) or late (greater than seven days after EDC). (6) CRH in maternal plasma at labour and cord blood were correlated (r = 0.29, p < 0.05) as were maternal and fetal beta EP (r = 0.43, p < 0.001). (7) Fetal obstetric difficulty was correlated with fetal beta EP (r = 0.54, p < 0.001). Our findings support the hypothesis that maternal plasma CRH regulates maternal beta EP during the third trimester, but other factors are involved during labour and in response to maternal obstetric stress.