It dates from the beginning of pregnancy, which is 2 weeks before conception. If you have regular cycles, that will line up with you LMP. But if your cycles are irregular, it won't line up. Your "due date" is 40 weeks from the beginning of your cycle, which if you have a 28 day cycle, is from you LMP. If your cycle is not 28 days, the dates will never line up. It is all guess work anyway, since pregnancies last from 38 to 42 weeks.
at 7:22 PM on Apr. 27, 2011