**1. Introduction**

The gestation in the mare begins with the fertilization of the ovum, then the implantation of the blastocyst in the uterus followed by the development of the placenta and fetus until delivery. Therefore, gestation is a dynamic and coordinated process involving systemic and local changes in the mare that support the supply of nutrients and oxygen to the fetus for growth and development in the uterus [1]. In part, these changes occur through the secretion of hormones in the placenta, which in turn interact with each other and exert extensive effects on maternal tissues during gestation [2]. These endocrine changes in maternal physiology adaptations to gestational status result from modifications in the maternal environment of steroids such as progesterone (P4), estrogens, androgens, and other hormones such as relaxin and prostaglandins (PG). However, an inadequate adaptation of maternal physiology can lead to gestational complications, such as restriction or overgrowth of the fetus and premature delivery [3].

Since an understanding of endocrinology in equine species is useful when considering hormone treatment of cyclic and pregnant mares, this chapter considers a basic review and applications of this information in clinical therapeutic situations. For this reason, this chapter aims to provide an overview of the endocrine changes that occur in the mare in response to gestation and to discuss the key role of hormones in mediating pathological processes.
