Apparently the earlier u/s have more accurate dates because they are before major growth spurts, BUT having said that most u/s are between 5-10 days of each other and is why they aren't always an accuate estimation for your EDD. Generally your LMP would be the best date to go by. I unfortunately don't know my LMP but know within a week, and my first u/s is 13 weeks, so I am just going by an estimate, I say that I am due at the end of October/early November. HTH!