I worked full time at the beginning of both of my pregnancies and I was sick in both of them.
I didn't cope all that well either I'm afraid.
With my first one I ended up telling my boss at six weeks because I was really ill and I didn't want to her to think my general apathy at work was the result of lack of interest or something.
The second pregnancy I didn't have to tell her because she picked it herself as soon as it started LOL.
My advice would be to be upfront about what's going on. That way they can make allowances and it won't reflect badly on you (especially since on prac you'll be working with people who don't know what you're usually like).