Baby’s Heartbeat!

So I was on Instagram earlier and i seen a post that said “This baby is 5/6 weeks! Around this time you’ll be able to hear the heart beat!”

I recently just had a miscarriage (01-04), and I went for my first ultrasound at 6 weeks and 5 days (I think.. 12-28) and we didn’t hear our babies heart beat.. and the doctor was saying that I was either 5/6 weeks he couldn’t tell.

My questions are:

1. when are you able to hear a heart beat? (I thought it was like at 8-11 weeks)

2. Could my baby have been gone at the appointment and I didn’t know?