There was a study done by Harvard I think some time ago, they found that first pregnancies were found to have a longer gestation period than second or subsequent. My two were both early, but the best date to go by is your LMP or last menstural period. Ultrasound is not 100% accurate and the most accurate scan you can have I think is if it's done at something like 7 weeks or earlier - as it's based on normal developmental stuff - as baby gets older, genetics takes over and baby grows at his / her own rate, so you get an EDD based on mathematics, which I think is crap because you end up with all these inductions as a result of mathematics. So if you have a bigger than average baby, they may induce you based on the fact of size and not proper development in baby's own time which is a worry.