It all depends on how your work will react (and which country you are in! hehe or in the US, which state you are in!)
My work is very very family friendly and my boss knew I was TTC becuase I asked the company to offer extra disability pay insurance. His wife is 8 weeks pregnant and gets really bad morning sickness so he's going to very sympathetic. I told him today and he was so happy, hugged me and told me I can work from home if my MS gets too bad.
Back when I told him I was trying he said we'd hire someone to cover for me while I'm gone, and when I come back, I can be their supervisor (so a promotion too.. though I know the timing with the company has to work out as we're starting up and need to be able to afford another employee and have the workload for them). He had asked to be told as soon as possible so he can start strategizing as I'm rather a key employee (the only accountant) so he wants to find the right person and know what our workflow will be near my due date etc. He told me about his wife so early because he said I'd be told even if something went wrong as he'd have to be out to support her. So I figured the same worked for me.
I think if your symptoms are noticable, you should tell them and nothing needs to be even planned for coverage until a minimum of 3 months before your due date! So if you don't have to tell them, you can wait (though some countries have a minimum notification requirement).