Due date question ?

Okay so I know the exact date I conceived and I know the date of my last period. According to both of these my due date is December 9th. Mind you I’m already 38 weeks today lol but when I was 7.5 weeks pregnant I had an ultrasound done and the baby was measuring 6 days small, so they changed my due date to December 15th! 🙄 I ignored it and continued to call it the 9th. But on all my paperwork they have the 15th! But according to them baby is always measuring a week ahead ......BECAUSE THEY HAVE MY DATE WRONG duh. So In their eyes I’m 37 weeks and in mine I’m 38 weeks. Which date should I honestly be going by ?