Why did dating scan change lmp by 7 days. which should i go by? i want to find when i ovulated to find paternity.

Merely estimates: In pregnancy dating, all dates for natural pregnancies are estimates (plus or minus 1 week). That means 7 days earlier than a presumed date, or the presumed date, or 7 days after the presumed date, ARE ALL THE SAME DATE. That's how it works. But, if a woman only had sex ONE time, she knows her conception was likely 1-5 days after that. Dating merely estimates conception within a 2-week window.