How common is it for my HCG not to double in 48 hours?

I went to my OB for some blood work and my hcg was already 3,026 at about 5-6 wks (sun pm). They had taken my blood Tues afternoon and it had only gone up a little over 800. A know the higher it gets, the longer it takes for it to double, but I also hear many women's don't in 48 and they have perfectly normal pregnancies. What do you think?