Comparison of changes in ovine plasma chorionic somatomammotropin concentrations in the fetus and mother before spontaneous vaginal delivery at term and adrenocorticotropin-induced premature delivery

Fetal and maternal plasma ovine chorionic somatomammotropin concentrations have been measured in eight pregnant ewes immediately prior to spontaneous vaginal delivery at 146 ± 2 gestational days. In seven additional ewes, fetal and maternal plasma ovine chorionic somatomammotropin concentrations were measured immediately before and during induction of premature delivery by infusion of 1 μg · hr^-1 synthetic adrenocorticotropin (ACTH_1-24) to the fetus commencing at either 120 to 122 gestational days (n = 3) or 130 to 132 gestational days (n = 4). Both fetal and maternal plasma ovine chorionic somatomammotropin concentrations fell significantly (p < 0.05) in the 5 days before spontaneous term delivery; no change in the fetal to maternal ratio for plasma ovine chorionic somatomammotropin concentrations was observed. In contrast, the maternal plasma ovine chorionic somatomammotropin concentration rose (p < 0.05) during infusion of ACTH_1-24 to the fetus while the fetal concentration declined. During infusion of ACTH_1-24 to the fetus the fetal to maternal ratio for plasma ovine chorionic somatomammotropin concentrations also fell. The finding that the parallel fall in ovine plasma ovine chorionic somatomammotropin concentrations in the fetus and mother in spontaneous term deliveries is not seen following ACTH-induced premature delivery suggests that mechanisms other than activation of the fetal adrenal axis induce the prepartum fall in ovine chorionic somatomammotropin secretion. These findings militate against a role for the fall in ovine chorionic somatomammotropin in the initiation of parturition.