Due Date

J
Have any of you ladies had your due dates changed? And if so, what made the date change?
I'm asking because every ultrasound I have had the baby has measured 3-5 days further ahead, and today at my 20 week scan, he measured a week and a half further. 
I KNOW what days we had sex and when I ovulated. I truly believe I'm slightly further than they are dating me. They are going solely on my last period date. I know large babies can cause delivery issues and sometimes require c sections, and large babies already run in both my hubby's and I's families, and I don't want a due date issue to cause a problem with inductions etc.