We heard our bub's heartbeat at 5wks6days because we went in to see our doctor as we'd had some spotting and needed reassurance. Thankfully, all was fine and there was a little heart flickering away in there!
It's the most amazing thing in the world to see that little flicker on the screen and know something's going on in there, especially as you can't feel anything happening ( except the overwhelming, all day morning sickness.... lol )