beta help

Anna
Hello all,
I have had numerous beta tests and the HCG has been rising well. 
My results have been
54
277 (3 days later)
749 (3 days later)
3896 (this was done 5 days after 749).
Today I took another one, 45 hours after 3896. My result was 6936. This time it didn't quite double? Should I be concerned. My doctor didn't seem concerned. I have heard the the higher the hcg gets the longer it takes to double. Still just wondering if anyone else has experienced this. 
I also want to add that I have a previous loss, so I'm very anxious. 
Thanks Ladies!