I’ll weigh in here too with a success story.
My 14 year old maiden mare developed placentitis at around 260-270 (I don’t remember exactly) and started bagging up. Since I was getting conflicting advice (the Repro center where I was going to send her to foal out said “that’s her milk vein”), so it was a few days until I called the local repro vet who diagnosed placentitis and put her on Uniprim, Regumate, and Pentoxyfylline; she did an exam and said she had to “push my filly’s head back down” because she was already moving up the birth canal! Yikes. She said there was already “partial placental separation”, so worse than your mare.
Of course I was beside myself with worry.
My mare was at my boarding facility and on night turnout - my BM at the time had no repro experience, and during treatment I worried daily that my mare would foal prematurely in the field and my BM would find them I had trouble sleeping and lost weight (naturally my husband lost his job and my mother got colon cancer the same spring.)
The weeks went by and my mare responded to treatment; the repro vet came back out to check her and said she was “80% healed”!, Hallelujah! Since I didn’t want to take any more unnecessary chances, I trailered her to the Repro center a couple of weeks early so she could be monitored (Foalert was put in and they had a vet on-site), and drove an hour to see her every day, graze her, groom her, watch her stomach, etc.
She started looking pretty uncomfortable at about day 320 - the filly was bouncing around (she had always been very active in-utero - my poor mare), and the vet said it could be any day. You know how it is with maidens!
We were hoping she would hold on until day 325, and she did; delivered a healthy filly (now 7 1/2) who is the light of my life <3 She is not a big girl (finished at 15.2), but the mare is 15’3" and the sire (Escudo 2) was barely 16’0 - I WANTED a small horse since I am myself small, so it worked out perfectly. The mare is very healthy and is competing successfully.
A thought on placentitis: with older mares, the vulva is often a bit sunken, and it only takes one bacterium to infect the uterus. My mare had a partial Caslick done after foaling (she tore a bit), but if I had thought about it, I would have noticed that her vulva was a bit tipped and sunken and asked the vets whether a Caslick would have been a good idea way earlier in the pregnancy.
In any case, don’t despair! If treatment is started promptly there is a good chance she will heal and hold onto the foal until it is “fully cooked.”
Big jingles for you and your mare, and please keep us updated!! Fingers crossed.