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Using a cortisol RIA carefully validated for specificity in fetal ovine plasma, we studied plasma cortisol time-trend changes in eight chronically catheterized sheep fetuses in the last 22 days of gestation before the spontaneous onset of labor. Best fit exponential type curves were drawn for each individual fetus to define the time before the onset of labor at which fetal plasma cortisol was rising at different rates. Fetal plasma cortisol increased at a rate of 0.5 ng ml-1 day-1 as early as 17.1 days before labor in one fetus, whereas the mean value for all eight fetuses occurred 11.8 +/- 1.0 (SEM) days before labor. The rate of increase doubled to 1.0 ng cortisol ml-1 day-1 as early as 14.4 days before labor in one fetus, whereas the mean value for all eight fetuses occurred 9.3 +/- 0.9 days before labor. These findings suggest that the regulatory mechanisms responsible for the prelabor increase in fetal cortisol production are activated earlier in gestation than we previously thought.

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