As long as you and your husband are excited and happy then tell her like you are excited and happy. I wouldn't make any excuses or make it sound like you are sorry about having another baby because you know she is disappointed.
I was really scared to tell my dad, because although I knew he'd be happy in the long run, I knew he would "not be so thrilled". But we aren't little kids and we can make our own choices. I don't think you should break it your mom gently. I'd tell her just like you told her about your other children and just how you are telling everyone else. I don't know if that makes sense, but if you acknowledge her disappointment she'll think she is right. I mean, she probably will anyway, but don't let her think she is getting to you. =)