Dating my pregnancy

So I know I should go from the first day of my last period which makes me 7 weeks right now but when I have my first scan they’ll actually tell me how far along I am right? It just confuses me when symptoms should start etc. If I’m actually only 5 weeks now does that mean morning sickness is starting from now for example or does all the advice go from start of my last period too?! Curious 😂