When I found out, I was so confused! But then I realized it's counted from the first day of my last period, and I felt so emotional realizing how my body was already preparing weeks before I even knew.
My mother explained this to me when I was confused. She told me to count from the first day of my last period, as that's how they did it in their times too.
Just use an online pregnancy wheel or a tracker app. Put in the first day of your last period, and it will give you your exact week and due date instantly.
Haha, technically you aren't even pregnant on the first day! It's just counted from your last period's start date because it's the easiest way to track.