I had just started a job (like two weeks earlier) that was supposed to be temp-to-hire. They loved me, I loved them, and it looked like full speed ahead in the getting hired for real direction. Then I got pregnant and was faced with a problem: I could tell them early and give them ample time to find another candidate for the job OR I could let them hire me and THEN tell them.
The second option seemed a little unethical to me, so I scheduled a meeting with my boss and when she said "how are you?" I replied "pregnant!" and we went from there. They were extremely accomodating in keeping me on the job - as a temp, unfortunately - and giving me any and all flexibility for doctor's appointments and days when I just couldn't drag myself there.
Once we got closer to my due date, I gave them a firm "last day" and all was well. Good thing, because my last day was a Friday and the baby arrived the following Monday.
So my advice - schedule a meeting if it's difficult to get your boss any other way. That way, you won't just be ambushing her when she is in the middle of six other things. Be upfront about doctor's visits and how you're feeling and what you might not be able to do and how you plan to deal with that. If you're planning to take leave and come back to this job, take a VERY active part in how to deal with your time off and who will replace you etc etc etc. As in everything, honesty is usually the best policy.
Good luck!
The Addams Family Motto: We gladly devour those who would subdue us.