It is possible to have an early due date with early menstruation, and the due date is usually projected from the last menstrual period. The due date is calculated from the first day of the last menstrual period, the month minus three or plus seven, and the day plus seven. However, if you cannot remember your last menstrual period, or if you are breastfeeding and are pregnant without menstruation, or if you have irregular menstruation and your periods are often early or late, you cannot calculate your due date according to the above algorithm. Instead, you should calculate your due date according to the onset of the early pregnancy reaction, the value of the HCG, and the size of the sacs and germs of the fetus as well as the length of the head and hips on the ultrasound.