A V belly isn't always completely noticeable, it too depends on the mare..
These are all photos of some of our girls on the day/night they foaled (within a few hours): (sorry not the best photos,, were taken for my reference)
Now out of those 4 the only one that I say screams "V belly" is the first one, just looking at her you can tell she would foal within at least the next 6-12 hours (foaled about 7 hours after that photo)..
The second mare pictured didn't have much of an udder prior to foaling and she surprised us totally.. I was on campus as it was graduation weekend, parents went out and did chores, check on the girls, no change in any of them, all ate their breakfast etc.. Then headed into to town to get gas in the car and pick up a few other things before coming down to campus to help pack up stuff and get ready for my graduation the next day (siblings were staying home that night and coming down in the morning so they could keep an eye on the mares, had a family friend going to check in on them while everyone else was down for graduation) anywho, was in town hour-hour and a half tops, came home to find our little Phoenix just born..
Third mare (BAD picture, but only one I could find from that night) kept us guessing for a few weeks, course was our first time foaling her out and only her second foal so her changes etc were not quite as noticeable as those of the more experienced broodmares that I'm used to.. She never really "dropped" much, but like I said, was only her second baby so her belly muscles hadn't been stretched as much as those broodmares that have had several babies, so she kept tucked up pretty good..
The last mare is the black pinto I told you about earlier..