Haven't posted much, but I've been watching your girl off and on. We had a mare down with an abscess a couple of years ago (long story, she'd had surgery on her other hind, was crippled, and the abscess just put her on the ground). Vet had her on antibiotics, pain relief, you name it for weeks and nothing helped. I found something called Thrush Off - look up www.well-horse.com for info. I emailed the company and the farrier who produces it actually called me. It's very different than the normal thrush remedies. Per his directions, we mixed it with sugar & gauze strips, packed her foot and wrapped it for a short time, several days in a row. Within a couple of days she was walking again and never looked back. It's all natural, not caustic and should be fine for a pregnant mare. It literally saved my mare's life. Sure worth a try if you are dealing with abscesses.
Jan