@solar_chase I never heard the reason given that if you do it, you WILL have a miscarriage (that'd just be superstitious nonsense, IMO); rather, I heard -- and all the women I know subscribe to it -- that the *chances* of miscarrying are higher during the first 12 weeks, so if you tell people you're pregnant before then, and you end up miscarrying, it'll be awkward to have to tell them all about the loss. So, these women reason, rather wait until you know "for sure", so to speak.
I doubt that includes your dentist, though. They're a medical professional, so they need to know these things. And strangers as well: you can tell a relative stranger. Just not close friends or family or... anyone whom you actually care about and who cares about you, I guess.