evillicious, i think you should inform your employer. better to know what they think now then to wait until probation and if they are not happy, they still have the right to terminate you. on the other hand, if they are pro-family, i think will be happy to accept the news.
I don't think companies will have a clause to say they will terminate staff when they're less than 3 months pregnant when joined..quite discriminary, but unwritten rules, maybe some will have.
just now i went out for a meeting with my boss and i disclosed the news to him. he was driving..then suddenly he said, "oh, now i have to be careful when I can driving" .haha..he congratulated me and he told me if i have morning sickness, I can come in late in the morning. =)