Well they won't sack you, if that's what you're thinking. They can't.
I think you're right in telling them sooner rather than later because, if you waited until after your probation ended, they'll think you've been a bit sly.
Just be positive, and tell them your fantastic news!
Probably a good idea to have some some of plan in place, which you could maybe mention. By that, I mean do you intend to return to work after the baby is born. I know it's early days, but I think this would probably be a good idea.
Good luck... and congrats! x