The effects of intrafetal ACTH administration on the outcome of pregnancy in the mare.

Enhanced adrenocortical activity in the fetus is related to the onset of parturition in many species. The aim of this study was to determine the effect of injection of fetal ACTH on gestational length and fetal viability in the horse. Pony mares (n=23) were studied from 300 days gestation. Seven control mares (Group 1) received three consecutive intrafetal injections of sterile water, while fetuses of a further 16 mares received Depot ACTH1-24. These mares were either allowed to foal spontaneously (Group 2, n=4) or delivery was induced within 3 days of the last fetal injection (Group 3, n=7); 5 mares aborted within 48 h of intrafetal ACTH injection. Maternal plasma progestagen concentrations increased significantly (P<0.05) in Groups 2 and 3 mares following intrafetal ACTH injection, compared with Group 1 mares. All Group 1 mares delivered live foals at a median gestational age of 327 days (interquartile range, 323 334), except one that aborted 16 days after the last fetal injection. Gestational length was significantly (P<0.05) shortened in Group 2 mares (median 314 [312-314]) days compared with controls. Group 1 and Group 2 foals showed normal postnatal adaptive responses and endocrine (plasma cortisol/progestagens) patterns. Group 3 mares delivered live foals at 306 (306-308) days gestation except one that aborted. Their foals were less mature compared with Groups 1 and 2 foals. Eight ACTH-injected fetuses were meconium-stained at delivery, including four that were aborted. Eight mares had thickened placentae, including three that aborted. The data show that both precocious maturation of the equine fetus and a significant reduction in gestational length compared with controls may be achieved in pony mares by intrafetal ACTH injections. This is probably mediated via adrenal regulation of fetal maturation and production of maternal progestagens.