How do you use a pregnancy calculator to calculate your due date?

A pregnancy due date calculator calculates a due date by counting 40 weeks from the day that the last menstrual period started, according to BabyCenter. This method assumes that conception occurred two weeks after the last menstrual period began, which is typical for women with average menstrual cycle lengths.

Sometimes, women may know the exact date of conception if they have been tracking ovulation or know which day fertilization must have taken place. In this case, a woman may enter the date of conception into a pregnancy calculator, and the due date is calculated as 38 weeks from the date of conception, according to BabyCenter.