My Wife Got Positive Result In First Blood Test A

My wife got positive result in first blood test a week earlier. HCG count was 81.77 IU/L. Then doctor advised to do a test again today, but result was 91.15 IU/L. Doc said that she is pregnent, but count suppose to be more. Asked again to go for an another test by week. Any hopes to being pregnent or what is the issue here. Pls advise!