The Maternal-Fetal Cortisol Gradient During Pregnancy and at Delivery1

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Abstract
Diffusible cortisol (DF) was estimated as total cortisol × per cent bound, determined by equilibrium dialysis at 37 C against an equal concentration of albumin.Maternal DF values in late pregnancy were about 2½ times those of non-pregnant women. In cord serum, DF levels correlated (r = 0.95) with the total cortisol levels and were higher at term than in early pregnancy. Amniotic fluid DF levels were similar to cord levels in early pregnancy but were only about ½ those of cord serum at term. A transcortin-like protein was demonstrable in amniotic fluid in early and late pregnancy. Maternal venous levels were higher than cord levels but if allowance was made for placental conversion of maternal cortisol to cortisone (estimated to be 80%), the cord levels were higher. Cord arterial levels were higher than cord venous levels, the differencebeing significant in the spontaneous-onset group. Cord arterial levels in the group with spontaneous- labor were higher than those of the combined induced and cesarean section groups although the maternal levels were similar in all three. These data accord with the concept that suppression of the fetal pituitary-adrenal axis is prevented by placental conversion of maternal cortisol to cortisone, that the bulk of circulating cord cortisol is derived from the fetus, and that there is a surge of cortisol in the fetus in associationwith spontaneous-onset labor which occurs independently of the mother.