EDC/EDD

normal lg ba na mag kaiba yung EDC (estimated date of conception) sa 1st ultrasound and 2nd ultrasound? In my 1st ultrasound kasi was Sept. 2, 2019 (mag fo-four months na tyan ko dun ko pa nalaman na preggy ako) and sa 2nd ultrasound was Aug. 20, 2019. thank you?