New job advice

So I just had a job interview yesterday. It went amazing and the job would be so perfect for me. I didn't mention the pregnancy in the interview and I'm terrified they're not going to want me. I know legally they can't fire me due to a pregnancy but if they offer me the job I was thinking of telling them right then. I have easy pregnancies and always work right up to when I go into labor. I just don't want to start this great opportunity on the wrong foot. How do I tell them?!