How do I tell my new boss i am expecting?

So here's the deal. I literally just started a new job on Monday, today Saturday I find out I am pregnant... I am so nervous and don't know what to say or how to tell him! I am a dental assistant and have to take x rays everyday... Which can be somewhat dangerous in pregnancy. I feel like I need to say something sooner rather than later.. He is an awesome dentist and is so nice and genuine, way better then the previous dentist I worked for.. Help!!!