OP, how old are you? I'm just curious. I know you intend to keep the foal if your mare is pregnant, but if you are just a teenager/college student/whatever, how do you know if you will be able to finance 2 horses? Less people want grade horses, even though I personally prefer them. I own an "odd" cross right now, whom I registered as half arabian. For some breed associations, only one parent has to be registered. As Britt said, you could register the foal with the Competitive Trail Horse Association.
There are many options, but sometimes the horse breeding business is a complicated place. My neighbor bred a half NSH half Mountain Pleasure horse, and it took 3 years to sell it. Purebred, registered horses are easier to sell, and if I was ever in a money pinch, I would end up being able to sell the registered ones a whole bunch easier. You really need to think about the foal in the long term, not just selfish reasons of wanting to keep it, etc etc. I personally wouldn't support a horse abortion, but I also wouldn't want my future foal to have the slightest chance of being neglected or ending up in slaughter.
This is not meant to offend you, I just really hope that you really think about this. Even she is pregnant and you decide to keep the foal, you BETTER update us with pregnancy pictures, hoo-ha pictures and foal pictures! ^_^ My best horse is a half arabian half rocky mountain, and he kicks all of the purebred horses asses at shows! (Excuse my language)