Serum cortisol binding capacity and cortisol concentration in the pregnant baboon and its fetus during gestation.

The cortisol binding capacity of serum from 11 pregnant baboons (38 samples) and from 7 baboon fetuses delivered prematurely or at term was measured after removal of endogenous steroids. Values for maternal serum collected between 60 and 120 days after mating (59.0 +/- 6.4 mug/100 ml, mean +/- SD) were greater than those for serum collected at term (42.3 +/- 4.9 mug/100 ml). The cortisol-binding capacity of fetal serum collected between 100 and 132 days' gestation was similar to that of the corresponding maternal sample, but at term was only 50% of the maternal value. The rate of clearance of cortisol from both fetal and maternal serum may therefore increase progressively during the last trimester of pregnancy. This effect is likely to be more marked in the fetus. The cortisol binding capacity of 15 serum samples from 9 non-pregnant baboons was 33.4 +/- 5.5 mug/100 ml. Mestranol2 (administered 200 mug/day im for 15 days) significantly increased the serum cortisol binding capacity. The concentration of cortisol in maternal serum from 7 pregnant baboons (10 samples) was 44.0 +/- 8.4 mug/100 ml and was independent of the state of gestation. In fetal serum the cortisol concentration was 4 mug/100 ml before 168 days' gestation and reached 49 mug/100 ml after normal delivery at term. These findings suggest that the mechanisms for production of cortisol by the fetus mature as gestation progresses. The physiological significance of the marked difference between the cortisol concentration and the cortisol binding capacity of fetal serum awaits elucidation.