**Author details**

*Animal Reproduction in Veterinary Medicine*

disease or insufficiency [172].

**5.5 Prostaglandins**

**6. Conclusions**

Relaxin is a useful biomarker to assess placental health and can be monitored in high-risk mares. Ryan et al. [172] reported a positive relationship between circulating levels of relaxin and poor outcomes in high-risk pregnancies. Relaxin is detectable in the blood after the 80th day of pregnancy without any changes until the second stage of labor. In mares with impaired placental function, in cases of placentitis, placental abruption, hydroallantois, and hydramnios relaxin concentrations decrease below 4 ng/mL [143, 172]. Low circulating levels of relaxin have been reported both in pony mares affected by fescue toxicosis associated with placental disease and agalactia and in Thoroughbred mares, with other forms of placental

In the case of placental hydrops, the risk of spontaneous rupture of the fetal membranes increases significantly [173]. Relaxin has been explored as a potential marker of treatment success in placentitis due to its level decrease in cases of spon-

Placentitis is characterized by the production of proinflammatory cytokines (such as IL-6 and IL-8) and PGs [175, 176]. PG release increases uterine contractility and consequently the risk of premature delivery [138]. Proinflammatory cytokines and the PGs of the FPU increases both in response to inflammation/infection, inducing premature activation of the fetal hypothalamic-pituitary-adrenal (HPA) axis [177], accelerating fetal maturation before parturition [138, 178]. The fetal adrenal produces both progestins and, once sufficiently mature, cortisol. Fetal cortisol, in turn, enhances placental and uterine PGs production, further enhancing uterine contractility and resulting in fetal delivery. Since the maturation of the equine fetus occurs later in gestation [137] this implies that placentitis or maternal disease could be devastating to the newborn foal. However, early fetal maturation likely counterbalances premature delivery and may help improve the chances for foal survival [138, 178]. The supplementation with progestin and PG synthetase inhibitor can maintain equine pregnancy in the presence of PGF2 insults [146, 147]. In addition, Esteller-Vico et al. [170] showed that estrogen suppression resulted in a decrease in circulating PGFM, which suggests that estrogens partially regulate PG production during pregnancy since PGFM concentrations were lower but still increased during the last trimester of equine gestation in letrozole-treated mares.

Knowledge of the physiological basis of the estrous cycle allows us to understand the interaction of reproductive hormones and the factors or events that interact in the cyclicity of mares. These basic studies have made possible the correct manipulation of the estrous cycle, the advancement of the reproductive season or the synchronization of ovulation. A great contribution in this sense has been possible through the description of the follicular dynamics and the study of the different

Likewise, the adequate interaction between the ovary, the placenta, and the fetus guarantees the secretion of the correct hormonal patterns necessary for a successful pregnancy. Measurements of progestogens, estrogens, and relaxin, among other hormones, are useful for monitoring the health status of the placenta and fetal viability. This is mainly because placental pathologies or fetal death are mainly due

structures present in the ovaries of the mares throughout the year.

taneously occurring and experimentally induced pregnancy loss [174].

**5.4 Relaxin**

**20**

Katy Satué1 \* and Juan Carlos Gardon2

1 Department of Animal Medicine and Surgery, Faculty of Veterinary, University CEU-Cardenal Herrera, Valencia, Valencia, Spain

2 Department of Animal Medicine and Surgery, Faculty of Veterinary and Experimental Sciences, Catholic University of Valencia-San Vicente Mártir, Valencia, Spain

\*Address all correspondence to: ksatue@uchceu.es

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
