First off has the mare had many foals in the past? I agree that you don't want the mare too fat, but a little thin is worst than too fat, in my opinion. If the mare has had two or three foals in recent past then she probably won't have too much problem foaling if she is a little bit over weight. If this is the first in many years then don't let her get more than a little over weight. Don't forget, not only does she need to carry the foal for 11 months but she needs to nurse it for 4 to 6 months after. That is asking alot of the mares system, bulk her up a little. I feed a good quality horse hay, in the northwest I feed a top quailty orchard grass, no alfafa. pregnant mares get a 14 to 16 percent protein grain. I used brand called "profile mare and foal" seemed to work well, 16% protein but not too hot for my mare. I worked her up to a full coffee can twice a day by her ninth month. Vits and mineral block a must. Also keep the horse fit through about 7 or 8 months, I wouldn't ride her but I would work her lightly. A fit horse will not get so tired during delivery, can't work them much after nine months though, just my opinion. After nursing for a few months you may have a hard time keeping weight and a good coat on your mare, don't let her get too fat, but more impotant don't let her get more then just a little thin. It may be hard to get her back in good shape if she gets a little thin, especially after the 7 or 8 month point. Finally remember that a 9 to 11 month pregnant mare is very fat looking.
Just my opinion,