Getting a new job

I got a call this morning for a interview at a really great pre-school. I'm going to school to become a teacher and I worked at a child development center for 4 years prior to moving to Louisiana. I've been looking for a job at a pre-school since I've moved here. I'm 5 weeks pregnant and I'm not sure when I should tell them - if I even get the job. What would your advice be?