One of the other things that can cause early udder development is fescue grass? Do you know if you have fescue in the pasture? If so, you should remove her immediately, as fescue is okay during early pregnancy, but is unsafe in the last trimester. If you're not sure about the fescue, I would switch her to alfalfa during these last few months.
Placentitis would seem unlikely if she got a clean bill of health for breeding, but I'm happy to know your in contact with the vet. Certainly wouldn't hurt to ask her opinion about placentitis.
Meanwhile, we'll keep praying she holds this little one for a couple of months!
She is absolutely GORGEOUS by the way!!