I was told bub was a girl at 17w but at 19w (and every scan since!) bub has very definitely been a boy. I was chatting to my m/w yesterday and she was saying that she sees it all the time. Just recently she had a couple who were told bub was a girl the entire way through the PG, they even had a special plaque made before the birth with bub's name engraved on it and then out pops a baby boy...