Plasma growth hormone concentration in the chronically catheterized ovine fetus during spontaneous term delivery and premature delivery induced by continuous intravascular infusion of low doses of adrenocorticotropin or cortisol to the fetus.

Fetal plasma growth hormone concentrations were measured in 15 pregnant ewes over the last 5 days before delivery. In five chronically catheterized pregnant ewes that underwent spontaneous vaginal delivery 146 +/- 2 days (mean +/- SD) of gestation, fetal plasma growth hormone concentrations fell from 124.6 +/- 44.1 ng X ml-1 5 days before delivery to 35.2 +/- 31.3 ng X ml-1 at delivery. In three fetuses in which premature delivery was induced by the infusion of cortisol to the fetus at 128 days of gestation, fetal plasma growth hormone levels fell from 195 +/- 19.1 ng X ml-1 to 70.7 +/- 25.5 ng X ml-1 over the last 5 days of intrauterine life. In seven fetuses in which premature delivery was induced with infusion of synthetic adrenocorticotropin to the fetus beginning at 120 or 130 days of gestation, the fetal plasma growth hormone level did not fall (175.6 +/- 75.7 to 158.9 +/- 60.1 ng X ml-1). Fetal plasma cortisol concentrations at delivery were significantly higher in the cortisol-infused fetuses (214 +/- 38.4 ng X ml-1) than in both control (94.4 +/- 33.7 ng X ml-1) and adrenocorticotropic hormone-infused fetuses (94.5 +/- 31.9 ng X ml-1). The fall in the fetal plasma growth hormone level in cortisol-induced fetuses may be due to the higher fetal plasma cortisol concentrations achieved in the cortisol-infused compared with the adrenocorticotropic hormone-infused fetuses in which no comparable decrease in growth hormone was observed. These findings suggest that there are significant differences in the fetal response to various experimental regimens used for the induction of premature labor in the sheep.