Why Dr leverage on 1st day of last period?🤔

Why Doctors normally add 2 weeks to your pregnancy timeline? I thought I should be at week 3 but Dr told me I am at week 5.. Given I had 2 miscarriages before, I feel that sometimes it made you anxious when the yolk sac or fetal pole not visible at that time the Dr thought it should be but you know it is too early. Anyone facing the same dilemma as me?