My own personal experience from many years ago. Following recommendations gave first and then 2nd  anti-abortion shot at 7 months gestation. Mare aborted with 24 hours. Chocked this up to just one of these unfortunate things. Gave another mare her 7th month shot the next year and she aborted within 48 hours. Lesson learned the hard way - never again. (This was a Fort Dodge product) If a vet had administered this shot I would have had some recourse. If I was ever again to breed a mare I would keep her isolated from any new contacts for her entire pregnancy and the only shots I would give would be to transfer immunity to the newborn. Good luck with your mare and make sure you have a store of colostrum on hand.
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- Frances C.
December 2017, Washington & California
Case history: https://ecir.groups.io/g/CaseHistory/files/Frances%20and%20Phoenix
Phoenix's Photo Album: https://ecir.groups.io/g/CaseHistory/album?id=12382