27 weeks and new job

I had an interview yesterday and the guy really liked me, he said he'd definitely be calling me Friday and we would do a sit down with his boss and HR. I have not mentioned I'm pregnant and due January 13th. I actually put my two weeks in at my current job and my last day is the 24th so I really need this position. I'm feeling guilty I didn't tell him and I'm considering mentioning at the second interview but also want to wait until I'm hired so they can't change their mind. I would tell them immediately after the paperwork of course.

What do you guys think, mention it or wait? I know I don't have to disclose it and they can't fire me after but I know they could come up with a reason not to hire me.