HCG didn't double

My HCG levels went from 6400 on Saturday afternoon  to 10854 on Tuesday afternoon. 
I just got the results, so I haven't gotten a chance to speak to my doctor. Is that an ok number at 5 weeks?
 I thought it was suppose to double every 48 hours. This was a 72 hr difference. I was expecting to see numbers in the 18000.