yep def dont worry about it. i remember being told the later in the preg the more room for 'error' - ie may measure above or below and a week either side would def be right. My OB said til about 12 wks it is more developmental (even better earlier) and so easier to measure for dates and after that the later you go the more genetics have to play a part (same with diff body parts measuring diff ages ie my DS had legs that measured about a week 'older' than the rest - he must have just had long legs - as i do - not that you could tell when he was born or now)