How to break the news to Mom.

I am 29 years old and this is my first pregnancy. Tomorrow the boyfriend and I are going to my mothers for lunch. I dont have a very close relationship with her. But we would like her to know the news. What are some good ways we could do this...with out her flipping out or worrying her to much since the relationship is still new. When the time is right, it is right. But i want her to be happy for me too.