When they calculate your due date, they actually allow for the two weeks pre-conception. Because your LMP is usually the most concrete date that most women can remember (with the exception of TTC'ers who tend to know when they conceived down to the second) it is the most useful "landmark" to base a due date calculation off. A normal pregnancy from conception to birth is 260ish days, give or take 14 days each side. But the due date calculators that are used are padded out by 14 days to account for the two weeks lag time from LMP to conception.

Hope this makes sense...