Hello girls, what schedule should we follow?

Hello girls, what schedule should we follow? Based on the period of last menstruation (from day one of your cycle)? Or based on the embryonic (from the day of conception)? I read articles where it's written 11-12 weeks, and then I have to go there... I don't understand. My ultrasound showed 3 weeks, but according to the gynecologist it's 5 weeks ??‍♀️??‍♀️??‍♀️