When to tell: I'm not really the right one to speak to about this because there are laws depending on what your job classification is. For example, if you work at a studio, there are protections for you so you can't be fired for taking maternity leave.
Now in other classifications, it can work against you. For instance, a camera assistant, may want to work as long as she can not feeling that her job is putting the unborn baby at risk. However, the production team may feel differently because the camera assistant is carry heavy equipment cases, etc., and may be worried about injury and a potential lawsuit.
My advice, talk to a union rep or someone in human resources.
How to tell: not really sure why you're asking me this. Most people get excited and say, "I'm pregnant," getting all glowy and stuff. Is there any reason why your boss wouldn't be happy for you or that you have to tiptoe around him/her? I'd need more details to be able to help you out with this one.
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