Plasma corticosterone during perinatal period in postmature rats.

In the fetal rat, plasma corticosterone concentration (PCC) decreases dramatically on the 20th day of gestation and remains low until birth even if the parturition is delayed until 23.5 days postcoitus. In the normal term newborn, there is an increase of PCC in the 1st h after birth. In postmature newborn rats this increase is not present but PCC can be increased towards normal term values if the adrenal cortex is stimulated with cosyntropin or lysine-vasopressin. This suggests that the lack of elevation of PCC in the postmature newborn is not due to the lack of responsiveness of the adrenal cortex but rather an impairment of the hypophyseal stimulation. The absence of PCC elevation at birth could contribute to the metabolic disorders observed in the postmature newborn rat.