When do you tell new boss you're pregnant?

Jessica
I know I've posted something similar to this before but I have no clue what to do. Today was my third day at my new job and so far I love it! But at the same time I feel like I'm carrying around this burden not telling my new boss I'm pregnant. I'm currently 5 weeks and 4 days. My first ultrasound is 9/29. Should I wait til then to say something?