First or four?

I’m confused of when your first week of pregnancy starts? I’ve read two different things and one says the first day of your last missed period is your first day of pregnancy, and one other said the first day of your last normal period started. I’m wondering because Im hoping I’m getting symptoms of period coming, not pregnancy... I had sex the 24th of March, and I’m supposed to start the 9th.. my last period was the 9th of March and ended 5 days later.