The gals and I at my ranch in California always had a running bet going, that the mare would hold it until the full moon, then she'd drop it ;) When is the next full moon anyhow? ;)
I have never actually bred my own horse before, but i have been around lots of pregnant mares that were ready to blow. Keep an eye out for the waxed nipples (which you are already doing) and feel the muscles on either side of her butt.. im not exactly sure what they are called, but the muscles on either side of the tail bone/dock. When a horse is close to birth she becomes very "loose" back there, and if you press on that area, it should jiggle a little bit.
If you are really honestly concerned, perhaps a call to the vet is in order =/
Good luck sweetie! I wish i could be of more help!