I'm thinking this too at the moment. I've got terrible morning sickness so want to tell her straight away (I'm only 6 weeks), but equally being considered for a new job as part of a restructure and think it could impact my chances (I'm sure they'd cover it up well enough that I won't be able to prove it though!)
My view generally is 12 weeks is reasonable if you can wait that long and don't need to tell them sooner. I also think its polite to tell the boss before you put it on social media and announce to other colleagues
I have told both Hr and my director today. They both thank me for giving them plenty of notice period (am 9 +2) so that they can plan ahead. Also I dont need to lie for my appointments now. I feel relieved. I strongly recommend
I told mine quite early on but I work in a hospital so some limitations as to what I could/ couldn't do! She's been great at keeping it confidential and it's meant I had time given for midwife appointment etc!