26w, 5d... just was hired... now what???!?

Ok so yes I hid my belly, and I was hired for a case manager/social worker position. I know companies are not allowed to discriminate but facing reality, they do... Paid Training is for 3 months (due date 1/14/17) starting this coming Monday! But how soon should I tell them I am expecting? I am required to hand in a physical, I was thinking to have my doc write out I have no limitations with this position, maybe that may help????!?!!!!?