Just take a look at the pictures I posted above of my AQHA mare her maiden pregnancy, she didn't look very pregnant, and went around 360 days. I didn't know any better then, so I wasn't watching her bag much, don't know what it looked like when she foaled or right before.
And, Diane's favorite pictures of Tana's maiden pregnancy. They are in the archives for this sort of thing, but posting here too. She did foal a normal 320 day pregnancy, or there about; but didn't look very pregnant nor ready to foal.
Pics are about 8:30am and right after lunch.