I can't offer much in the way of advice, but i thought it was a general rule that the earlier the ultrasound, the more accurate the EDD was. We conceived DS2 "by accident" (no AF due to BFing DS1) and thought I was around 8wks when it was picked up, but the first ultrasound put me at 6 weeks with an EDD of 12/12/09. DS2 was born 11/12/09.