Can a ultrasound be ten weeks off?

If you are in the early stages of pregnancy &-8 weeks it will only be off by a day or two. By twelve weeks may be 3 days, 18/20 weeks 4-5 days. From then on it becomes more difficult as babies end up different sizes. Not at an early stage. Are you further on than you thought or not as far on as you thought? Give a few more details and I can tell you a bit more.