Scheduled C section momma's

I'm really confused, going off my lmp I'm 13 weeks. But from ultrasound I would be 14 weeks. I'm measuring a whole week ahead pretty much. From my lmp my due date is 10/11 but from the scan it would be 10/5. My ob said we will just stay with the 10/11 date but now I'm nervous. They schedule it a week early for a reason, so wouldn't we go off the 10/5 date to be safe?? Especially since I have big babies. My first was 10 pounds. I'm just kinda freaking out. To me that's a huge difference, and I dont want to go through what I did with my first.