hcg levels, high could it be twins

Okay ladies so I have a question , I am 9 weeks as of today at 6 weeks 2days I had my hcg level checked and it was already 29,665, I went in for an ultrasound at 7 weeks and 1 day and they said they only seen one baby but that level seems pretty high for at that time. What do you think could it be twins and the other baby is just hiding?