8 weeks slow heartbeat?

I am 7 weeks, 6 days and just saw the heartbeat today but it is very slow (roughly about 100) and embryo is only measuring about 6 weeks. I was tracking and using ovulation kits.

Doc said to prepare for miscarriage. Anyone experience this? Is miscarriage inevitable? Anyone end up with healthy pregnancies, or would the baby be impacted even a successful pregnancy?