Dating of pregnancy

So even with baby #3 the dating part confuses me. According to my last period I am currently 4 weeks pregnant today. But technically didn't get pregnant until 2 weeks after my last period. So I was only considering myself 2 weeks. When we talk about being able to hear the heartbeat around 6+ weeks that goes off the the dating of my last missed period correct? Just want to make sure I understand this. I didn't find out until a few weeks after my missed period with my previous two so I guess I never really paid attention to how the dating of pregnancy went