HCG beta levels. Anyone had the HCG rise, but not double and have a healthy pregnancy?

Danielle
I am currently 4 weeks 4days.  I had my blood drawn on the following days and these were the results.
Monday (3 weeks 6 days): 236
Wednesday (4 weeks 1 day): 577
Friday (4 weeks 3 day); 868 (I didn't quite hear the nurse when she said this number, but it was in the mid to high 800's).
I'm a bit nervous since it more than doubled the first 48 hours, but only rose by about 50% the next 48 hours.  
Should I be concerned?  I had a miscarriage last July and a still birth last March, so I'm just really scared about having another lose.  Thank you.