Generally speaking there is a distinct change in "muscle tone" around this time of year for mares due in April or May time.
I find it much harder to discern my "Old Timers" bags than I do the maidens.
Bagging up proper will start around 13 weeks before foaling, but Minis can wait longer than that....often!!!
I have had mares with virtually no bag at all foal and come into milk with no problem at all.
But I have never had a mare with no bag development at all do the same...when I say "No bag" I mean she has not filled completely and is not swollen, I do not mean there is no bag present.
I have seen pictures on this forum of mares that have absolutely no bag development whatsoever, no tone, no let down , nothing.
This is when I feel sure the mare is either not in foal or the dates are radically wrong.
There are no hard and fast rules about this, but there are obvious safety issues and it is not the norm for a mare with no udder development to foal.
It is, at least as far as my mares are concerned, the norm for a mare not due until May to start relaxing her bag and "getting ready" around now.
I must stress that I handle all my mares udders from now on,(*edited on instructions from mares....WARM YOUR HANDS!!!) every day, which gives you a comparison, this is not looking at the bag we are talking about, here!!
You need to get up close and personal, in the same way as you do with descending testicles.
Happy Christmas!!