Ultrasound dates use gestation age which is based on lmp not on conception correct ? so i couldn't have gotten pregnant before the date that was given

Maybe: EDD are based on 40 weeks from last known normal menstrual period, or 38 weeks from known conception date.U/S dating is based on a database of average fetal anatomic measurements (size of head, abdomen, leg etc) for a particular age.The computer in the ultrasound machine calculates an EDD based on the average of these measurements. In the first trimester, U/S dates are accurate within 5-7 days