Some of mine have never waxed at all, I've even had them foal with a very small udder for it to become huge after foaling, so best just to get your deck chair out and a heap of coffee and sit and watch!!!!
My mare is due VERY soon. She has a big belly and no wax. She has a small udder. I have been told another 3 weeks due to the fact that she is not " bagging up" But, I let that slide by cause not all mares wax!
Maidens are tough to predict. They don't read the book. Watch for nervousness, and for the nipples to be filled out all the way to the end. Also a loose tail, easy to lift. But then again, you may see no signs. They love to fool you.