Other than more feed, what are some considerations to think about when caring for a pregnant mare? Do you need to stop riding her right away? What about vet procedures? What does he need to do during this time?

Yes, one of the precautionary measures you can take it to stop riding her as soon as you know that she's pregnant. It'll help you a lot if you have her checked by a licensed vet. That way you'll get prescriptions for vitamins she can take to help her all throughout the pregnancy.

Definitely talk to the vet before giving her any vitamin supplements though.

There is a condition called milk fever, which occurs in lactating animals when they put out too much calcium, so that there's not enough calcium in their blood for their muscles ro work and their heart to pump. (yes, can be fatal). A lot of owners thing that they can stop this by giving calcium supplements during pregnancy. WRONG

In many cases the mother cannot eat or absorb asmuch calcium as she puts out, so she pulls some out of her bones. She's fine to do this and youc an supplement her while she's milking, but not before when she's pregnant. If you give her extra calcium when she's pregnant then her parathyroud glands (the things that allow her to pull calcium out of her bones) will get smaller from dissuse, and she wont be able to switch them back on fast enough when she starts milking.

I would add in make sure you give the horse proper exercise, but not too much. I think the same would apply for a horse as a human, correct?