Anatomy scan

Went with my husband to get my anatomy scan done and everything looks good thank God! I'm 18w2d and have had 3 ultrasounds prior to this one and at all of them I have measured right on track with my November 24th due date, but today I'm measuring a week a head they are not going to change my due date, but should I be concerned? Or is my baby just big enough now so it is growing at its own pace and I can't rely on the ultrasound anymore. I've read a lot about it and it seems to be pretty common to start measuring a little ahead or behind around this time so just seeing if anyone has any experience with this?