As all the other girls have said, don't worry too much about it unless your Ob tells you he thinks he is particularly concerned. Not only can ultrasounds and measurements be inacccurate at times, but everything is based on averages. Once your bub is born, you will find when you see the child health nurse your bub is also weighed up against even more measurments and averages. When reality is, we are all different shapes and sizes, so it seems silly to me when we have healthy babies. Unless you notice other health issues or are particularly concerned, discuss it with your caregiver.