Due Date differences

Is anyone else's baby measuring a few days larger than LMP? My sweet baby is due 12/25 based on my last menstrual period but with each scan my bub measures bigger. At my 12wk 1day scan, my bub was 12wk 6days.  Is anyone else experiencing this? Should I worry? Which day do you go by? My last baby measured a week behind and I lost him at 14 weeks. I'm a lil nervous.