i would see how the interview goes, if you are able to relate to the interviewers and you feel as though you are building up a rapor, i would mention it at the end when they ask you if there are any questions. just be honest and say, thank you for the opportunity to come in today, i have enjoyed visiting the premises and as i really hope to be sucessful and start off on a good note, i want to be honest with you and let you know i am pregnant, leave it at that and if they ask you about it, be honest.
if it's going a bit ropey or there is a dogey interviewer, say nothing and just wait and see.
my sister in law got hired as a dental nurse when she was 5 months pregnant, she had turned down a job at a local practice as she had hated the dentist and they got called for this interview, she wasn't really showing and as she got on well with the staff, she was honest and started the following week, 8 years, and 2 more kids later she is still there.
good luck