Actually it is 'slab' sided and it happens when a mare is very close to foaling. During pregnancy a mare's tummy gets bigger and wider as time goes on. As she gets very close to foaling, the foal should 'drop' down into the delivery position and if you look at the mare from the side or the back, it will appear that she has suddenly 'grown' a large lower bulge to her tummy from the foal's new position, which then makes the area in front of her hips look 'empty' and flat - hence 'slab' sided.
I dont have any pictures of my own to show you as my mares are always on the tubby (fat!) side and therefore dont really show any difference - although I can feel it! I'm sure plenty of the other Aunties here will have pictures they can post for you. But look carefully at the pictures because a slab sided mare will still have plenty of 'flesh' on her topline/back and should not be confused with a mare who can have a similar look but is undernourished or short of groceries (she will not be carrying the weight on her topline either).
Try checking on Amy's thread about Cinnamon - she's recently posted some pictures that might help you - the one from the rear really shows how much the foal has dropped.
There are also some helpful photos and videos on the pinned Foaling Video thread. Not the actual foaling videos but there is link that Eagle posted which is fantastic. That one also shows how to fix a simple dystocia where one leg is back. There is also a series of photos that JAde10 posted that shows body and udder changes. Tons of useful info on that thread and of course the foaling videos themselves are helpful.
Target, thanks for pointing out Jade10's wonderful photos. As a first timer, I feel that it shows exactly what I needed to see. I have watched several of the videos over last few months. And I still don't feel prepared.
I do have a question about washing her udders. When should I do this and what with? Shes good about being messed with there, shes not maiden and was bred by a breeder for her past foals, so I am sure she is expecting it, maybe even wanting it at this point.
I bet you could use a drop of mild soap and water but I just use a gentle setting on my hose (plain water ) w/sponge. Peanut always loved it and would actually spread or legs. Holly couldnt care less as long as she had her hay. There maybe reasons you should not clean ( in the wild, horses wouldnt ) but if you are gentle and they dont mind I see no reason not too. Maybe near the end they may be sensitive. The experts will jump in and tell you their thoughts I hope.
I would just use a warm damp cloth to wipe her udders with, making sure that you also get right up into the gap between the two halves - most mares like to feel clean and can get really quite uncomfortable with the dirt that can collect.
How far along is Diamond - and please do send your pics to Diane, we'd love to share in her progress.
The first is a collection of 3 in one. That one is progressive over 8 weeks. To see the belly dropping. And some pretty shots (gotta brag about my pretty girl