My due date moved around a bit in each pregnancy because babies don’t all grow at exactly the same rate. A difference of a week or two between your period dates and what the ultrasound shows is actually very common and usually nothing to worry about. It can just mean your baby is on the smaller side or that ovulation happened a little later than expected. The best thing you can do is eat well, rest and trust that your care team is watching the growth. If they are concerned they will let you know, but most mums with a two week difference go on to deliver healthy babies.