Skye, from the time I found out I was pg with DS2 I ended up having three ultrasounds (the second due to a small amount of bleeding) - the first at about 6 weeks, the second at 8 weeks and the third at 12 weeks. The first and third scans had my due date at 12/12/09 and second scan had him arriving on 11/12/09.

This doesn't exactly address your query, but I guess I'm saying that in my case, the u/s were correct. Maybe you should get another scan for your piece of mind.