Slow Rising HCG

My levels are not doubling in two days, much less three. However, my doctor said she isn't concerned. I go back Monday for another draw and we will go from there, but everything everywhere says they should be doubling so...I'm confused. The only thing giving me hope is that with my daughter, my levels were only 23,000 at 8 weeks (should have been at least 32,000) and they said she wasn't viable but she's 7 now! Here are my numbers:

10/11: 139

10/13: 208

10/14: 243

10/15: 270

Also, I'm 4 weeks 5 days.