Have you had a positive pregnancy test yet? If you have, then it is based on the start day of your last period. of the "40 weeks of pregnancy" the first two you aren't technically pregnant yet because of the way they count it. I know. it's dumb to me too. If you go see a doctor that can maybe explain it better.
The doctor usually bases it on your period. Unless you know the exact day of conception. I would go talk to your doctor, they will be able to give you an ultra sound to know for sure. I hope your day is lovely :)