I had a viability scan at 6 weeks and 1 day (according to my lmp) and saw a flicker of a heartbeat. Embyro was measuring smaller though than dates suggested and heartbeat was only 71bpm which was deemed a little low. But it was early.
Heard heartbeat on the ultrasound at 8 weeks 1 day. 167bpm. Was very exciting!
I hired a doppler which gives the heartrate following the NT scan I had at 12 weeks and tried it out after I reached 13 weeks - was concerned it would be too early but I found the heartbeat and it was such a relief in the weeks leading up to the 19 week scan when the worry would set in. I'd read about the disturbance that the doppler causes the bub, so I've used it as little as possible. I still haven't sent the doppler back, JIC, and it was again a relief to have it there as I did get worried a couple of times when I'd had lots of fluttery movement and then nothing for what seemed a couple of days.
HTH