How exciting! Foal watch!
We are relatively new to horses (how much longer before I can say we are not, I wonder?) and on our fourth foal (two mares). Skippa is "due" in six weeks. Her first foal arrived five weeks prior to her "due" date. I chronicled their udder development, and each just started stretching the udders almost exactly one month prior to birth. Keep in mind, one day the udders look large, and the next, totally shrunken. Some of it is edema, I suppose, which is helped with mild excercise.
Sometimes they are large in the morning and less so at night. As they get closer, they stay pretty full and tight
You mentioned white stuff, which may well be waxing, and jelly butt, another sure sign. When the foal moves into the birth canal, they do lose that pretty, fat round belly, and get rather slab-sided, and start looking rather bony as the foal drags the tummy down.
Your mare sounds like she may be very close, but they are so very unpredictable, she may keep you waiting for a couple more weeks!
Can't wait to hear the result!