Heartbeat with Doppler

Katelin
I had my first OB appointment on Friday at 12wks+1. My doctor tried to find the baby's heartbeat with a Doppler for a couple minutes but couldn't. She told me not to worry, 12 weeks can be early.
She scheduled another appointment in two weeks and will try the Doppler again and if still unable to locate the heartbeat will have me get an u/s.
Has anyone else had this happen? Should I be worried about a missed miscarriage?