If you know she'll be happy for you and that she is a source of support for you, definitely tell her! It will make you feel great, and she'll feel honored to have been trusted with such a precious gift!
While I understand, on one level, the whole "waiting to tell" thing because of concern of m/c, I also think anyone you'd tell who you know would be happy for you would ALSO be a great source of comfort if anything bad were to happen, so I personally only ever wait to tell people who I think WON'T be happy for me.
Unfortunately, in my case, that means I can tell all my friends and acquaintances right away, but I wait to tell my family, b/c they have not been supportive in the past of our choice to have a large family.
It's their loss, but I don't need anything to bring me down right now, so I'll wait a bit...
Enjoy your meal with your mom and sharing such wonderful news!