I heard my baby's heart beat at 11 weeks 4days. Pretty strong :-)
I was 12 weeks when they finally did the Doppler and could hear my daughter's heart beat. I previously had a miscarriage around 7 weeks and so this time that was something I was very admit about hearing. Hang in there it should be soon.
As long as you can SEE there is a heartbeat, then all is fine. It just takes a little longer to hear it sometimes.
I'm 10 weeks and I still haven't technically heard my baby's heartbeat.
I've had two ultrasounds, one was about 4 days ago and I saw the heartbeat.
If your doctor is saying it's there I wouldn't worry. He isn't going to give you false information.