My baby was measuring 5 weeks and 3 days at 6 weeks and 1 day, and had a heartrate of 80-83. Stupid technician told me there was a high chance of miscarriage as I'd already had some bleeding (it turned out that I lost a twin). I had some cramping a few days later and got a scan at the hospital, baby was all caught up (had grown from 2mm to 6mm) and heartrate was 120. In the early stages, the presence or absence of a heartbeat is more important than how slow/fast it is. The angle can make a big difference and early measurements are generally + or - 5 days anyway. They're looking for a heartbeat in something the size of a grain of rice, there's going to be mistakes. I had another scan at 11 weeks and Tiger was measuring just fine and jumping away in my belly. It can work out ok, out of all the women that go through a scare - some may lose their baby but there will ALWAYS be others who get good news. Trust that you'll get good news.