I learnt quite some time ago that if I wanted to keep things from my sons then I had to keep it from the husband too! He could never keep it to himself. I'm sure she wants to support you, though, having said that. I remember as a daughter being really flummoxed when my mum didn't tell me she was undergoing tests for diabetes and she said "Wait till you have children then you'll understand" and now, of course, I do.
Taz x