I know HOW to tell him....but WHEN to tell him??

So I'm worried this bean isn't going to stick because I just started taking thyroid medication barely two weeks ago. was not expecting it to work this well this quickly so I doubt I'm in the clear yet. I am going to see my doctor on Thursday and am working late. My husband has no idea and is working early. Thursday is also my birthday so I kind of want to surprise him in the morning before he pulls any birthday stunts... but part of me wants to wait until after I talk to the doctor on case it goes chemical. Sooooo WHEN do I tell him??