While your vet has stated it's not something to worry about with your mare, if I had a mare with unusual or excessive edema I'd be calling my vet. My mare in the past has gotten edema just ahead of her udders due to lack of exercise because unfortunately she had to be stalled the last month of pregnancy. If she had been left to be her normal active self and had gotten the edema or had the edema spread further than it did I would have called my vet. Generally if I were unable to explain excessive edema or edema placed in an odd place such as the chest I would be precautious. I don't know, it's just something I would freak out over just to be safe rather than sorry.