Canine parturition: what is known about the hormonal setting?

Parturition is a challenging physiological process with perfect timing dictated by the events leading to the end of pregnancy in the female, and by the maturation of the fetus(es). The process of parturition remains an intricate interaction of hormones in a fine-tuned timing that remains to be better elucidated in the dog. In the dog pregnancy is maintained by the progesterone production by the corpora lutea, in which some hormones play a luteotrophic action. At term of pregnancy, the pre-parturient luteolytic cascade is the most apparent event, characterized by a rapid decline in plasma progesterone concentrations and useful in predicting the onset of spontaneous parturition. In contrast, cortisol plasma concentrations measurement showed high variability and suggested to be related to the stress condition instead of the onset of parturition. Both prostaglandin F2α and E2 concur in the process of parturition in the dog. The measurement of oxytocin plasma concentrations, also very variable, is implicated in uterine contractions. The measurement of plasma oxytocin concentrations showed to be useful to distinguish between normal and disturbed parturition, allowing the prompt recognition of dystocia and the immediate obstetrical intervention. In contrast to other species, no significant roles of estrogens for the initiating of parturition were demonstrated. Relaxin, the main pregnancy hormonal marker in the dog, beside an endocrine action, is also supposed to play a paracrine/autocrine role at the utero-placental unit and to support the maintenance of high plasma progesterone concentrations through a luteotrophic action, concurring to the main action of prolactin. Although important information has been provided, some aspects in the understanding the hormonal interactions and action timing implications in the process of parturition in the dog remains to be better investigated and represent intriguing topic for basic knowledge and applied research.