After DS was born I did heaps of reading and some people think scans in late pregnancy can go as much as a kilo over or under. 4 weeks before my god son was born his mum was told he was too big and would have to come out soon. A week later they wanted to give her steroids injections because they though he was too small and his lungs might be underdeveloped. They can never be 100% sure. Don't worry too much just enjoy the last few weeks of your pregnancy and relax as much as you can.