ABOUT ULTRASOUND ACCURACY AND DOCTOR'S READING?

Hi, may grlfrnd is 10weeks pregnant now accrding to her ultrasound test but if we follow the date of our last contact the baby should just be 8weeks or 11weeks.. however its impossible that the baby is 11weeks old bec she had her LMP on dec4., is it possible that the doctor's reading was wrong or the ultrasound is not that accurate? I NEED YOUR HELP HERE.