LMP and Ultrasound off by 3 weeks

Had the same situation and had to return about slightly more than a week later for another scan. At the next scan, it was confirmed that I'm about 2 weeks off from my LMP! Could be due to longer menstrual cycle (say >30 days) instead of the average 28 days cycle so it is not accurate to rely on LMP to estimate duration of pregnancy. Try not to worry so much!!