**5.4 Relaxin**

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 disease or insufficiency [172].

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 spontaneously occurring and experimentally induced pregnancy loss [174].
