In several mammalian species, the fetal adrenal cortex plays an important role in the spontaneous onset of labor. No conclusive evidence has yet been presented that a similar mechanism is active in humans. Although in some previous studies the cortisol concentration in human cord blood has been found higher after spontaneous labor than after induced labor, it has not been possible to determine whether this rise in the fetal cortisol level precedes the onset of parturition or is a consequence of labor. In the present study, starting at the earliest possible stage of labor, fetal scalp blood samples were taken sequentially during 16 spontaneous and 13 induced vaginal deliveries. Cortisol levels in these 2 groups of samples were compared with each other and also with cord blood cortisol levels in II patients undergoing elective cesarean section. The initial fetal cortisol levels did not differ between groups with different modes of labor onset. A pronounced rise of plasma cortisol levels occurred during labor in simultaneously sampled maternal and fetal blood. These results do not support the concept of a role for the fetal adrenal cortex in the initiation of labor in humans; they invalidate the use of cortisol concentration in cord blood for the estimation of prelabor fetal cortisol level. The origin of the cortisol surge in fetal plasma during labor and also whether the fetal adrenocortical function responds to the stress of labor are discussed. It is concluded that the rise of fetal cortisol levels during labor might mainly be a reflection of the maternal response to stress.
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