Congrats! You should add 40 weeks to your last menstrual date & that should be your due date. When you have a detailed scan it will tell the due date more accurately.
It's the first day of your last period. Count from then onwards. That's the day your doctor will ask for anyway.
Try using a pregnancy calculator. you can find these online. Will give you a good idea of your edd.
Doc usually count last day of your PMS.
Try to use clear blue then it will indicate
Vany