My understanding is that genetics dont tend to kick in until after 12 weeks in-utero. That said babies still undergo differing growth spurts at different times so a scan is only a guide. Is your cycle regular? Like a 28 day cycle? Cause thats going to throw things around too. If you go by the 'old' calculations from the 'Every Woman' book your EDD is 18th January, so that throws a whole other ball into play Dont worry to much about it hun, its only an estimate and bubba will come when he/ she is ready. The main problem with EDD is being pressured into induction when you arent really overdue as such.