The estimated date of delivery. The due date is also called the estimated date of confinement (EDC). Calculation of the EDC or due date is performed by counting forward 280 days (40 weeks) from the first day of the last menstrual period. Most women do not actually conceive until about two weeks after the first day of their last menstrual period. Therefore, for the purposes of calculating the EDC a woman is already 2 weeks pregnant when she actually conceives. The due date is only an estimate and only 1 in 20 actually deliver on their actual due date. However, most deliver within a week of their due date.