Growth scans can be really inaccurate, but a bit of a guide is that if the measurements are all similar, ie all measuring 2 weeks ahead and not all different, it is more likely to be accurate. The genetics of the parents come into play the further into the pregnancy, so if dad has a big head then baby might too or if mum is long and lean baby might be too. So this effects growth scan measurements. Also the skill of the sonographer and the body habitus of the patient play a role in the accuracy.