Saw two heartbeats at 6 weeks with a transvaginal u/s.
They can usually see a heartbeat starting the 6th week of pregnancy...but like sarge said, it has to be a transvaginal ultrasound.
:-)
A transvaginal ultrasound would probably work better at being able to see a heartbeat at this early stage.
A really good technician and most doctors can see the heartbeat this early, but usually are not alarmed if they cannot see it because it can be very difficult to find. Good luck!