I told my boss at around 7 weeks as I'd had two episodes of bed rest due to spotting and due to my m/c history the doc took this seriously. I just felt it was best to warn him that I may have to have more time off because of it. As it turns out I didn't have to take anymore time off so I probably didn't need to tell him so early.

Even giving my boss so much notice my replacement only started 2 days before I finished up (as planned) so make sure you give them plenty of time to recruit your replacement.