Is it possible that you may have ovulated later in your cycle, hence why you are measuring behing.

With this pregnancy of mine going by my LMP my due date is the 24th November. When I had my scan at 12 weeks it put me due at the 18th November then I had another scan 2 weeks later due to complications and it put me due 21st November. I actually agree with this as I think I ovulated earlier in my cycle, plus I know when DH and I DTD as it was only once and it was a week prior to when I was meant to ovulate, hence why I'm sure I ovulated earlier.

Gee I hope that all makes sense.

Only other thing is you baby might be a bit smaller, but given the consistency of your earlier scans I think you may have ovulated later in your cycle.