I didn't have HCG bloods done so I don't know what my levels were like.
I had a scan at 6+4 and there was a sac, yolk sac and very VERY tiny fetus. Then had another scan at 7+4 and there was the same and the sac looked irregular. Lost the baby that night.
Like I said, though, it really is 50/50. For every woman who lost their baby there will be one that just had a slow starter