When should you tell boss your preggo?

I'm only 6 weeks going on 7, I just started a new job which the pregnancy won't interfere interfere in anyway. However since it is new business they have set up a date for employees to come help and move equipment inside. And it's a gym so it's going to be heavy stuff. I feel like I should say something before that date comes up so they know I won't be able to lift. But I also feel it's way too soon. Any thoughts? And any tips on how I should go about telling them?