Magic/Gin I believe what Magic said is correct. From my own experience, in my first pregnancy I saw a HB on scans at around 6 and 9 weeks, but baby had died by 12-14 weeks (dates a bit unclear as I hadn't had a proper dating scan due to lost hospital referral). Of course, you can sadly lose a baby at any point in pregnancy, but up to the end of the first tri is definitely much higher risk. And, as Magic says, the dating scan also looks for healthy development, rather than just a HB.
My philosophy is to tell people about the pregnancy during the first tri so long as I would also be comfortable telling them about a loss. I ended up telling lots of people about my MMC later on, even if they hadn't known I was pregnant in the first place, just because it was such a big thing for me and I wanted to be honest. At the time I was going through it, it was also invaluable to have support from those who knew. But everyone is different. It depends what you are comfortable with.
Elsa
for you. Sorry you're having such a tough time.