Likelihood of missed twin during ultrasound?


I have had 3 pregnancies (1 miscarriage) and I am 10 weeks today. Even though this is my second baby, I feel larger than I should be - I also had elevated hcg levels from blood work at 6 weeks. My first ultrasound only detected one baby, but what is the likelihood that a twin was missed? Anyone have any experience with this? Or am I just going to be massive with a singleton? lol