Michelle, one of the ladies in the January (2006) pg buddies thread had a low heartrate on the first scan. She ended up having a baby that was just fine. Couldn't your dates be a little out? If you are a couple of days behind what you think, then the heartrate might be slow just as the bub 'gets warmed up'. The heartrate starts a bit slow, then speeds up during pg only to slow again at the end.

I hope all is just fine with your little'un