I think if you have 2 EDD that are within 2 weeks of each other then they don't mind so much? I remember when they done my 20 week scan they said they will confirm my due dates with my first scan that was done somewhere else and the doc said that as long as they are within 2 weeks of each they will be happy. At my 22 week appt they told me that my due dates matched perfect and will be staying the same. But it seems alot of due dates change.