Tess, although I will agree with your comment most of the time, there are times that mares will not bag up to any real degree until the foal is born.. But generally a would say you are correct. ALTHOUGH I had a mare that bagged AND waxed to just show me how good she was at false labor. She is a rescue that we believe had her system flip it's foaling switch as soon as it got here with some great eats.. she dried back up and we are now on a waiting game.
Guess bottom line is when it comes it comes.. great things to look for but no fool proof stuff..