2nd interview for a dream job

I got a 2nd interview for a job that is kind of a dream job. I interviewed once and did not disclose pregnancy, because I didn't feel comfortable in doing so yet. I was only 4 months, so it wasn't obvious. It's been 3 weeks, and I just got a call back for a 2nd interview and a meeting with a board member! Good sign. I am now 5 months, but can still hide it with flowly clothing. My gut tells me to wait to tell them I'm pregnant until I get an offer. I know I am not legally obligated to tell them, but the ethical lines are so blurred here that I'm torn. What are your thoughts? To tell in 2nd interview, or not to tell? I don't want it to influence their decision in any way.