Should i go by lmp or dating scan at 9 weeks? dating scan changed lmp date to 6 days later than it actually was. is this because i ovulated late?

That's the same date: In pregnancy dating, all dates for natural pregnancies are estimates (plus or minus a week). That means 6 days earlier than a presumed date, or the presumed date, or 6 days after the presumed date, ARE ALL THE SAME DATE. That's how it works. They are the same day. But, if a woman only had sex ONE time, she knows her conception was likely 1-5 days after that. Dating scan at 9 weeks is accurate.