My OB decided to keep my due date the same from their original call (September 25th) and I'm almost…

My OB decided to keep my due date the same from their original call (September 25th) and I'm almost positive when I conceived and it puts it at September 21st. I know it's not a big difference but my baby is measuring at 20 weeks 5 days as 21 weeks and 5 days. Today would make me 21 weeks from my dates... But my baby boy is measuring big for his actual age. Since I delivered at 35 weeks with my first, I'm just worried about him surprising me like my first did lol.