Studies on fetal, neonatal and maternal cortisol metabolism in the mare

Fetal plasma cortisol concentrations (mean +/- S.E.M.) in ten animals with indwelling umbilical catheters ranged from 13-9 +/- 1-5 ng/ml (227 to 244 days) to 18-1 +/- 2-2 ng/ml (290 to 310 days). Maternal values did not change over this period (15-9 +/- 1-7 ng/ml). Fetal cortisol production rates in two fetuses were 3-6 and 3-8 mg/kg/day at 291 days; there was little placental transfer of cortisol. In three fetuses (319 to 321 days) plasma cortisol values 1 to 10 days before birth were higher (30 to 50 ng/ml) than in any other group, and at birth the values were comparable with those found in intact Thoroughbred foals at birth. After birth plasma cortisol concentrations fell rapidly in most foals, but very high values were found in two foals with postnatal convulsions. Plasma cortisol levles remained low in two Thoroughbred foals born prematurely: similar values were found in one catheterized foal both before and after parturition.