Unbound unconjugated cortisol in umbilical cord and corresponding maternal plasma. Materno-fetal gradient: comparison of methods.

Unbound unconjugated cortisol was measured in cord venous and arterial plasma and in simultaneously obtained maternal venous plasma by equilibrium dialysis at 37 and 5 °C and by centrifugal ultrafiltration at 37 °C. Total cortisol was quantitated by a specific method based on chromatography and competitive protein binding. 67 cases were studied at cesarean section, 7 at vaginal delivery. Regardless of the method used, values of unbound cortisol in cord plasma were found to be much lower than previously reported. The values for unbound cortisol in the mothers exceed approximately 3-fold those in the newborn.