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!!

Normal! If you want to count your weeks of pregnancy based on LMP it will never be accurate even if you have a regular period. That's why your EDD follows the ultrasound which is more accurate. Nothing to worry about!