If a woman has regular cycles there will be a 2 week gap between lmp and ovulation. The due date
for you will be calculate by
your LMP even if your conception date
is known as that is the way it has been traditionally done. The traditional method assumes the two week gap or greater if the cycles are varied in length. (It is just easier to ask/know when your last Lmp was then to know when one ovulates). That is where the utility of the US measurments really shines.