i went for my 12-week scan at (i thought) 14+5 weeks but they put me back to 13+2 weeks, although on my maternity notes they put my date as june 12th which would hav made me 13+0 weeks at the time. it is confusing, but i guess the baby will come when its good and ready anyway, it dont matter what date ur given! (unless u hav an elective c-section)
my sister and one of my girlfriends said their dates were put back too, but their babies came early (so it seems their periods were right all along!)
the good thing about being put back tho is that u got extra time to be considered overdue so less chance of being induced