In obstetrics we use the last
menstrual period to date the pregnancy which adds two
weeks to the embryologic gestational age. Ultrasound calculations are programmed to correspond to the lmp dates to avoid confusion. SO, if your LMP puts you at 7 weeks, then you are 7 weeks pregnant, and your due date will be 33 weeks from now.
It's just a lot easier to go with the LMP dates!
Good luck!