Even early scans aren't always accurate. I think this is more because not every woman has a 40week pregnancy. For some women there baby is just as baked at 37weeks as someone who is 42weeks pregnant. Without inductions may women would even go 43-44weeks pregnant. With my last DD my early scan at 7weeks gave me a EDD of 15th May. She arrived spontaneously 19/4 at 36+3 looking not the slightest bit early (no vernix, fat etc) and weighing 8lbs. I would offer early scans especially when you don't know your dates but when looking at baby's actual birth date to the early scan there is often weeks difference. I think they come when they come some earlier some later.