Hi raving-monkey.
As puma said, bet that was defo a shock!
Personally I think she should have told you about it, but, as you mention, when would have been a good time? I would imagine there would have been a time she could have brought it up in the past 6 months you have been together.
Anyway, I would find it very very hard to trust her after that but you know her alot better than I do and only you can figure out if you can trust her again.
I hope everything works out ok and congrats on the baby! :)