Confused

Hey mums. This might be a silly question and I'm sorry if it is but I was talking to the Dr and the ultrasound lady and I don't get why they go off the first day of last period and that's how many weeks you are. So I'd be almost 10weeks...

I mean the first day of your period you're not pregnant then, only after your period when you have sex... I'm so confused and again I'm sorry if this is still but I don't get it. I'm 26 and I still don't get it😂😂