I guess we wait and don't wait, in a sense. With my first PG (a loss around 6-10 weeks*), we told my parents/brother and BFF around 5.5 and 6.5 weeks and then told my extended families by proxy that I was PG and probably m/c around 8 weeks when we got the concerning u/s. We had planned to just share the happy news at that time (Passover/Easter-- we decided not to go under the circumstances).
This time (I was lucky enough to get PG again pretty quickly), we waited a little longer. Told my parents right away (3.5 w) and brother and BFF at 5.5 weeks, because they are really our support system. We did wait until just this weekend (12.5 w) to tell anyone else-- DH's parents and siblings. We will tell my grandmothers/extended families at 13.5 w and slowly tell other close friends and whatnot... Probably won't "announce it to the world" (FB, coworkers) until more like 16 w.
I don't feel like it's exactly because we had the loss that we're (sorta) waiting... Like, not so much afraid of having to possibly "untell." But we definitely felt less anxious/were less chomping-at-the-bit this time. I don't think 12-13+ weeks seems like "waiting"-- but apparently a lot of people do tell earlier, and it has been a bit strategic on our parts, so I guess sorta-waiting.
I can totally understand waiting until 20+ weeks, though-- especially if you won't be showing! I am not really showing yet, unless you know me REALLY well. In fact, the way I'm built, I don't think the general public would guess for a while. But by 20 weeks, I imagine I will be-- if not to the general public, then certainly to anyone who has met me before. I guess we'll see, though!
I was surprised at how I kinda like/don't mind having a little "secret" though. I am the kind of person who can't keep her mouth shut and tells the whole world her business, so surprised I haven't felt that anxious to tell. It's a partial relief to be able to start discussing it openly and not have to hold my tongue about some things (and fake-drink), though.
*Babe stopped growing around 6.5 weeks, we found this out, and that I might m/c around 8 w, confirmed at 9 w, m/c between 9-10.5 w.