irregular period how many weeks ?

i found out i’m pregnant and i wanted to know if your period is irregular how do they know it’s accurate to do it from the first day of your last period ? like the lady told me on the phone you are about 8 weeks(first day of last period was sep 10) but how do they know if i didn’t actually conceive in september maybe i conceived in october because my period is always late anyway. sorry if that’s confusing but not sure how accurate it is to do it from period date. thank you.