Pregnant Mare Care
So you are going to raise a foal.
Congratulations! You are embarking on a miraculous journey, one that usually has a happy ending. There are several things you can do to avoid the most common problems. First ask yourself some questions. How old is your mare? Is she in good body condition, not ribby or thin? Has she had a foal before, and if so, were there any problems? If there were difficulties, or you are unsure about your mare's overall condition, be sure to discuss them with your vet.
SUPPLEMENTS:
Years ago my vet recommended feeding my broodmares Mare Plus, a supplement made by Farnam. Most feed stores and tack stores carry it, as well as TSC and some WalMarts, or you can order it from Jeffers or Omaha Vaccine at a lower price. Keep her on it thru the pregnancy and while nursing the foal; in fact, all year round if you plan to rebreed her for the next season.
COMMON CAUSES OF ABORTION and HOW TO PREVENT THEM:
Rhinonuminitus--I always vaccinate my broodmares at 5, 7 and 9 months of pregnancy for Rhino. Be sure that the vaccine you buy is Pneumabort or another that is specifically for the strain of Rhino that causes abortions in pregnant mares. The regular Rhino/Flu shot you give horses to prevent upper respiratory infections will NOT PROTECT against abortion.
Fescue--Make sure your mare does not get any fescue grass or hay (hay is rare) the last 60 days of her pregancy, it carries a spore that can cause abortion, difficult births, and a lack of milk in the mare and no colostrum. I have never lost a foal or mare from this, but have had a couple mares that did not have any milk, until my vet and I had the mare on meds for several days. They had been pasture-bred and I was off on the due dates, and left them out too long in a pasture of mixed grasses, including fescue. Made a believer out of me, and now I pull them off AT LEAST 60 days ahead of due date.
Black Cherry trees and webworms--It appears these may be the culprits in the "abortion storms", stillborn epidemic and new foal deaths last year in Kentucky. Remove any black cherry trees near your horse habitats, and kill webworms with an approved spray. Contact me at meriwalker@msn.com for references if you want more information on this hazard.
Twinning: Twinning in horses is more common than most owners realize. It is also a primary cause of re-absorbtion of the fetus by the mare. (This section is under construction.)
Supplements like Mare Plus; Rhino shots at 5, 7 and 9 months to prevent abortion; Pull mare off of fescue at least 60 days prior to due date; Control growth of black cherry trees and webworms; Ultrasound for twins early in pregnancy; Vaccinate with annual vaccines 30 days prior to due date;
Worm 7 days prior. 
Mare care is a topic near and dear to my heart.
I love my mares and foals. So this may be long and more info than you want! But it's based on years of personal experience, attending equine reproduction clinics at UGA Vet School, working with other breeder's, and from working with a great vet who specializes in equine reproduction. I work to stay current on new developments. That said, remember every situation is different. I am not a vet, and do not intend for this information to replace your vet's advice. I am not covering every possible difficulty you may encounter with your pregnant mare. But I can give you some tips based on what has worked for me. The health of the mare during gestation is key to the development of the unborn foal.
The mare needs to be a healthy weight, not fat and not thin. Moderate exercise is good, in fact I used to ride my mares when they were in foal up to about the 6th month. A frequent question is what to feed a brrodmare. I like Manna Pro's "Broodmare and Foal Developer" for the last trimester and while lactating. The amount to feed really depends on the individual horse, and any change in feed must be done gradually to prevent colic. This feed is great for the foal as a creep feed, too. For hay all mares and foals get free choice coastal bermuda, and once a day the mares get "Mare Plus" by Farnam.
Manna Pro has recently added more supplements to their SuperHorse line of feeds, like biotin. I think they are the best company for improving their feeds as new research comes out.
If the hay quality is not great, I add a scoop of alfalfa pellets or soaked beet pulp to each broodmare's feed. If the mare's a hard keeper or I'm trying to prep the colt, I add 1/2c. corn oil to the feed. Gives them dapples and even more shine, and helps add weight without the danger of colic. Vaccinations
Around 30 days prior to her due date, vaccinate the mare with all the usual annual vaccines you give, like EWV and Tetanus, Potomac Fever, WNV, rabies, whatever you normally give in your area. This allows the mare to pass on antibodies at their fullest potency in the colostrum (first milk) to the foal. If you do this, you may not need to give the foal a tetanus shot or vaccination until age 6 months. Worming
Worm the mare on a regular basis, and again at 7 days prior to due date. I use ivermectin type wormers almost exclusively. (If you have not wormed a horse for a long time, it may be best to worm first with Safe Guard or Panacur, and then 2 or 3 weeks later again with ivermectin.) The foal will not pick up parasites as quickly if you worm shortly before the mare foals. Foals inevitably eat some of the mare's manure, to get the natural bacteria and enzymes they need to jump start their digestive system, and they pick up worms that way. So, a quick summary:
Good quality feed and moderate exercise;
If you want some info on what to do during or immediately after foaling, follow the links below (under construction 2/7/03). Again, these are things that have worked for me and are not intended to replace advice from your vet.