The amnion is a layer of the foetal membrane that has mechanical protection function and allows expansion and growth. The diseases that affect amnion have rarely been reported in horses. Amnionitis and funiculitis have been implicated as components of abortions as observed in abortions caused by the mare reproductive loss syndrome. Here, we report an abortion at 240‐day gestation of a 7‐year‐old Anglo‐Arabian mare with a history of previous stillbirth. Blood samples from both mare and foetus were collected for serological testing. Fragments from the placenta were taken for macroscopic and histopathological examinations. The results of these tests ruled out leptospirosis and concluded that the cause of the abortion was non‐infectious chronic amnionitis. Moreover, the decreased vascularization in the placenta was consistent with placental insufficiency. This case highlights the importance of monthly ultrasound monitoring in equine pregnancy in order to diagnose placental insufficiency. The study also confirmed the efficiency of the histopathological examination for the definitive diagnosis of placental inflammation and for the study of foetal vascularization to rule out placental insufficiency in equine reproduction.