I was having a clearout of my 'favourites' in my browser and came across a couple of links from babymed.com and was thinking that this may help someone in early pregnancy.
This first one is about normal hcg levels, and gives an estimate on events ie yolk sac appears, heartbeat appears and embryo appears.
normal hcg levels
And the next one is information about hcg levels.
hcg information
I hope that this helps someone. I know how hard and distracting hcg levels can be in early pregnancy.