Most vets will advise you to take your mare off of fescue pasture 60 to 90 days prior to foaling and to provide her with a "dry lot" area to prevent a "red bag" delivery. A dry lot is a field or paddock that doesn't have grass growing in it. You must then feed her hay so she can continue to graze but be sure it does not contain fescue. You may want to consult your local Agriculture Dept. to see who grow hay in your area that does not have fescue grass in it. It's hard to find sometimes so you may have to go to alfalfa which is very good for the mare's milk supply.
Here is a good link for you to learn and study about the effect of fescue pastures and bred mares. If you do a google search, there are many more links that will come up for you that are very informative. Good luck with your new foal.
http://www.arabians-international.com/fescue1.html