HCG help!!


I had a miscarriage in June so my doctor is watching me close this time around. First HCG test was 3,300 at 5 1/2 weeks 4 days later it was 13,00 4 days after that it’s 27,00 and eight days after the last one it’s only 82,000 and I’m 7 weeks... why didn’t it double? Is this normal because I’m 7 weeks?