Qualitative and quantitative assessment of the circadian rhythm of cortisol in pregnancy

The effect of pregnancy on the circadian rhythm and diurnal excursion of plasma cortisol and urinary free corticoids was examined in a sequential study during the second and third trimester and 6 to 12 weeks post partum. Hourly blood samples from six subjects and 8-hour urine collections from eight subjects were obtained around the clock. While the circadian rhythm was maintained during gestation, plasma cortisol levels (24-hour mean, nadir, peak, and nadir-peak excursion) increased. The relative excursion of plasma cortisol (expressed as the percentage of deviation from the 24-hour mean) exhibited remarkable blunting compared with postpartum values. This pregnancy-associated blunting of plasma cortisol excursion was indicated by a significant reduction in the: (1) mean peak and nadir excursion, (2) integrated area between the percent deviation curve and the 24-hour mean, and (3) mean slope of the major incremental and decremental segments of the percent deviation curve. The circadian rhythm and diurnal excursion of plasma cortisol were reflected in urinary free corticoid values. Mean 24-hour urinary free corticoid concentrations increased 180% during gestation over nonpregnant levels. Nadir concentrations of urinary free corticoids in pregnancy exceeded peak nonpregnant levels. The gestational rise of metabolically active free cortisol and adrenocorticotropin (ACTH), and the pregnancy-associated blunting of the excursion of plasma cortisol may be explained by an autonomous source of ACTH during gestation.