Effects of delivery on fetal unbound cortisol concentration.

Non-protein-bound (free) cortisol in plasma is that fraction of the total plasma cortisol concentration that is generally regarded as biologically active. The concentration of free cortisol in cord vein plasma obtained at delivery was found to be related closely to the total cortisol concentration. After vaginal delivery following spontaneous onset of labor the free cortisol concentration in cord blood was not significantly different from that measured following induced onset of labor. The free cortisol concentration in the cord blood correlated significantly with the duration of the second stage of labor. These changes in the free cortisol concentration in cord plasma, rather than constituting a primary event in the onset of labor, may reflect the stress of vaginal delivery on the fetus.