I wonder what on earth is wrong with your mom. The only thing that I can think of is that she still thinks that she "owns" you. Perhaps you have not acted enough like an adult up until now, letting her tell you what you should and shouldn't do, letting her think that she is still "parenting" you? Perhaps the umbilical cord between you two has been very short up until now?
If this is kind of how things have been between you two, then I think she feels betrayed that you are all of a sudden so totally NOT under her thumb. What she is doing to you now is just terrible. I'm very sorry. If you two don't have this kind of relationship I am describing, then I can't figure out what on earth has come over her. It isn't like you are 17 years old. It's not like you had a one night stand. I hope that you and the boyfriend are close enough to each other and love each other enough that marriage might be on the horizon.
At any rate, I would cling to the other set of parents right now. I wouldn't try very hard to deal with your mother at this point. She needs time to come to the realization that she is really going to be a grandmother. After the baby comes, I do think you should go over and introduce her to the baby. If you decide to marry, you could invite her to the wedding, or if you two elope, just tell her when you show up that you and your BF decided to tie the knot with the justice of the peace. Don't make a big deal out of it.
If she is SO cold as to walk away from that little baby after seeing it, then you know that this is NOT you and that it really IS her. Don't beg and plead and crawl all over the place trying to get her back in your life.
I wish you the best. I hope she realizes what a mistake she has made. She would be lucky if you could get over what she has done to you, quite frankly.
Dawn