At 6 weeks they usually don't see a heartbeat. I had a scan at 6 weeks 5 days and it could be detected then so when you go for your next one it should be more accurate.

With my m/c's the bleeding was lighter than a normal period, no cramps and some small clots, so it doesn't necessarily have to be a large blood loss to m/c, but some women bleed heaps and go on to have a normal pregnancy. It's a very stressful time, please take care of yourself.

Good luck.