HELP!!!

I have a job interview today for a county job working at the police department dispatching, I've taken and passed all the tests this is the last step. I'm pregnant I don't know if I should tell them or just wait until after I'm hired. I am desperate for a good job I don't want to jeopardize it because I'm pregnant. Should I tell them? If so when would be a good time to mention it. I want to work as long as I can until the baby is born and continue after but I'm just nervous they will not hire me once I tell them at the interview. I've worked so hard to get to this point please help.