Can sonograms be wrong?

I took a sonogram today and it says i am 7 weeks 5 days, but according to my lpd i should be 8 weeks 6 days, and i know that i am right on my lpd, cuz i keep track. So can it be wrong? I am pretty sure i am further than 7 weeks 5 days cuz i have a lot of symptoms and feel that the sonogram is wrong! could it be?