Increased exposure of tissues to cortisol in late pregnancy.

In late normal pregnancy the mean day-night variation of non-protein-bound cortisol in plasma was found to be reduced to 52%, from a mean of 82% in non-pregnant subjects. Levels of unbound cortisol at midnight and at 9 a.m. in late pregnancy were raised above the non-pregnant normal. These levels led to increased urinary free cortisol excretion in pregnancy.This "tissue exposure" in late pregnancy is two to three times normal, as judged by both plasma unbound and urinary free cortisol measurements. The well-known increase in urinary free cortisol excretion in pregnancy is explained by the reduced day-night rhythm.