From the first day of your last period. Not from when you would have been ovulating. From day 1 of bleeding of the last period you had before getting pregnant. All medical counts use day 1 of bleeding as the start point for a pregnancy. If your cycles are irregular, the ultrasound's computer will compute the start point by looking at the size of the embryo, but will still count back the two weeks from the assumed conception, to get to the average day 1 of last period.