I'm not sure what the earliest is hon but don't be too shocked if they can't find it as that is quite common. I had a fall at 20 weeks and went straight to my OB for a checkup. The midwife used a doppler but told me that she wouldn't be suprised if they couldn't find the heartbeat on it even at 20 weeks. We did hear it thumping away immediately which was a huge relief.

Just try not to worry if they can't find it. The position of your placenta has a lot to do with it too and you won't know where your placenta is positioned until one of your major scans (12 or 20 weeks).

I hope you do get to hear that beautiful galloping sound though. Good luck!