Twenty-four–hour pattern of cortisol in the human fetus at term

Objectives: Indirect evidence suggests that adrenal steroid production in the human fetus may have a circadian rhythm. To assess whether there is a 24-hour rhythm of fetal cortisol in the human fetus, we investigated the relationship between fetal and maternal cortisol and cortisone concentrations in maternal, umbilical arterial, and umbilical venous blood samples over a 24-hour period. Study Design: Elective cesarean sections were scheduled every 2 hours around the clock in 57 term (38-41 weeks’ gestation) nonlaboring pregnant women. Plasma cortisol and cortisone concentrations were measured by high-pressure liquid chromatography. Results: The mean 24-hour cortisol concentration was higher in umbilical arterial than in umbilical venous blood samples, 63.6 ± 4.6 ng/mL (SEM) versus 48.7 ± 3.2 ng/mL, respectively (P