Don't worry too much about the 20wk scan results in terms of EDD. They generally don't use that as a guide for working out EDD anyway and for some women they will have a lot of variation for each measurement. I remember with one of my 20wk scans the measurements were all over the place and there was about a 10 day variation between the lowest and the highest gestation. It is less than a week that the dates are out by, so just stick with your original dates.