Here's Boo right before she foaled:
Boo is so freakin' huge in the body that she never looked as down in the belly as your mare does. But then, she didn't even really look pregnant until the last month before she foaled. Her udders though.....She was a Jerseymaid COW. I think you can see that she was loose in her tailhead area in the bottom pic.
ETA, see the sweat on her side? She was already in labor when I took that pic.