I'm not going to say what you should do either way, but I will say, if you feel that you can do this, then don't give up the job offer just because you're pregnant. I was offered a full time job when I was 10 weeks pregnant (it was just a casual thing prior to that). I accepted, then thought, we'll I'll do the right thing and tell them. BIG MISTAKE! Not only was the job offer withdrawn, I was also fired.