Adrenocorticotropic hormone, cortisol, and progesterone changes in the lamb during the perinatal period

Although the changes in circulating concentrations of adrenocorticotropic hormone, cortisol, and progesterone are well established for the fetal and neonatal lamb, there is little information on these hormones in the immediate perinatal period. We have examined the relationship between these hormones and systemic blood gas tensions and substrate concentrations in the perinatal period. Measurements were made in arterial blood of seven unanesthetized fetal sheep at 138 to 141 days' gestation during low- and high-voltage electrocortical activity. After cesarean delivery each newborn lamb was studied again at 2, 5, and 10 minutes and at 2 and 24 hours while awake. There was no correlation between fetal or maternal adrenocorticotropic hormone, cortisol, or progesterone and fetal electrocortical activity. Within 2 to 5 minutes of delivery there was a dramatic increase in neonatal immunoreactive adrenocorticotropic hormone concentrations associated with an increase in plasma cortisol. Neonatal Pao2 rose progressively, but a mixed respiratory and metabolic acidosis was evident during the first 10 minutes after delivery. This was partially corrected by 2 hours of neonatal life and was fully corrected by 24 hours of life. We conclude that rapid responsiveness of the fetal pituitary-adrenal axis occurs in response to birth in the absence of active labor, and we suggest that this may have survival value for the preterm fetus.