The establishment and maintenance of pregnancy in domestic animals requires interactions between the developing conceptus and the maternal system. These interactions are essential for maintenance of the corpora lutea (CL), conceptus development and placentation, regulation of uterine endometrial secretory activity, placental transport of nutrients and gases, regulation of uterine blood flow, achievement of immunological "privilege" for the conceptus, stimulation of development of the maternal mammary glands and various other effects on the physiology and endocrinology of the maternal and conceptus systems. Interactions between the conceptus and maternal systems are also responsible for initiating parturition. All of these events are discussed, on a comparative basis, with respect to current knowledge obtained from studies of domestic farm animals and, where appropriate, data from laboratory species.
