Good for you for taking in a horse in need
I would highly suggest reading up on a lot of basic ground manners. Obviously since the mare is pregnant, you wont be doing much riding at first. Some of the most important things about owning a horse (basics) are as follows:
1. A horse is NOT a dog. If you bought it to feed it treats and just let it get away with everything, you WILL have a handful.
2. Horses are herd animals, and they will constantly test to see if you are their leader, or their subordinate. Do not be afraid to let them know who is boss
3. Hitting a horse will not kill it. Obviously you don't want to beat the thing, as they are a LOT stronger than you will ever be, but you need to make sure the mare understands that you are her leader.
4. If you EVER doubt you can handle something, call someone to help you or call a trainer that can give you pointers.
5. Nipping is not cute. Dont let a horse nip you. Thats not a cuteness in the horses mind. Its a way to dominate you. And it can quickly turn to biting, which can crush/break bones. NOT CUTE!
6. Horses have a short type of memory, like dogs. Therefore it will not understand why you are punishing it for something it did 5 minutes ago.
I follow a rule set out by a trainer named John Lyons. If a horse bites, kicks, pins his ears, or turns his or her butt to you in blatant disrespect, you have 3 seconds to make that horse regret it. (RULES: NEVER hit a horse in the head, never ever do damage to the horse, and stop after 3 seconds)
7. Get an equine vet and a farrier set up NOW. The last thing you want to be doing is looking for a vet/farrier in case of emergency. Have both and talk to them tellling them of your situation. They have priceless info and can help greatly in the care of your new horse.
8. Read the forum, especially those dealing with horse diet and basic respect/training. There is a lot to learn but it can be done. Everyone starts off as a beginner at some point.
9. Get some horse friends to help you out. Its always easiest to learn from someone personally rather than on a computer.
Sorry for the novel, there is so much to learn. I just put some basics of horsemanship that I thought important to remember. You will be okay as long as you do your research, ask questions, and take proper care. Remember, ignorance is no excuse on why a horse is not taken care of properly. YOU are responsible for her and the baby.
Good luck and keep us updated