I would do what you were planning on doing. It would be nice to give her a heads up beforehand so she isn't blind sided at the party. I know you've already offered support, but maybe try again in the email just to let her know that you're there if she needs someone to talk to who has been there. I'm sure it will sting, but she needs to find a way to be happy for you (or at least pretend to be ;) because you deserve this baby and this time of happiness and anyone who has struggled to get where you are should know that. I used to hate it when everyone around me would tell me they were pregnant followed by an, "I'm sorry" or "I didn't want to tell you ..." It used to make me feel even worse.
P.S. I remember seeing your ticker when it said, '3 weeks and 5 days pregnant' and now you're in the 10's already! Time is flying!