Pregnant and starting a new job

Hi! I just found out on Saturday that I'm pregnant. I'm 27 and unmarried. I accepted a new job two weeks ago that I start in a week. I'll be 7.5 weeks pregnant when I start. Has this happened to anyone else? I'm so nervous. I know they can't fire me for being pregnant, but I'm also hired on a 90-day probationary contingency, where they can fire me for basically any reason. What should I do? Should I wait the full twelve weeks to tell them and just work my butt off until then?