Maternal dexamethasone treatment in late gestation induces precocious fetal maturation and delivery in healthy Thoroughbred mares

Abstract

The foal requires an active hypothalamo-pituitary-adrenal (HPA) axis for organ maturation and post natal survival. Prenatal administration of synthetic glucocorticoids may provide an effective method for inducing fetal maturation safely in the mare. To determine whether dexamethasone administered to late pregnant mares: 1) will induce fetal maturation and precocious delivery; 2) is safe to use and 3) to identify endocrine responses in the mare and foal. Pregnant Thoroughbred mares received either 100 mg dexamethasone i.m. (treated n = 5) or 50 ml saline i.m. (control n = 5) at 315, 316 and 317 days of gestation. Plasma progestagens, cortisol and prostaglandin F(2α) metabolite (PGFM) concentrations were measured before and after treatment. The foals were weighed, the crown-rump length (CRL) measured and an adrenal stimulation test performed on Day 1. Dexamethasone significantly (P<0.01) reduced gestation length in treated mares without apparent adverse effects. Plasma progestagens increased (P<0.05), and cortisol and PGFM (P<0.05) decreased, following dexamethasone treatment compared with control mares. Foals were clinically mature but those from dexamethasone treated mares had reduced (P<0.05) CRL, but not bodyweights, compared with controls. Their cortisol concentrations increased following exogenous adrenocorticotrophic hormone stimulation but 2 foals from dexamethasone treated mares showed evidence of adrenal suppression. Dexamethasone stimulates precocious fetal maturation and delivery in healthy late pregnant mares. However, fetal HPA activity may be suppressed. Dexamethasone treatment could be used to improve foal viability in mares at risk of preterm delivery. The endocrine effects of such a therapy must be evaluated before clinical intervention with glucocorticoids can be recommended.