Heartbeat and dates

So I've had 3 scans so far (history of miscarriage) if I go off the first scan I'd be 10 weeks 3 days, if I go off the second scan I'd measure 10 weeks 6 days and if I go off the scan today I measure 10 weeks 4 days. The heartbeat the last time I went around 8 weeks was 174. Today it was 164. Should I be concerned that the heartbeat was lower and that the scan dates are all a little different?