The due date is calculated based on the first day of the last menstrual period. The formula is that the due date is equal to the date of the first day of the last menstrual period plus seven, and the month of the first day of the last menstrual period plus nine or minus three is the month of the due date. For example, if a pregnant woman’s last menstrual period comes on January 11, her due date is, 1+9=10, 11+7=18, which is the due date of October 18. Since the time of ovulation is not completely fixed for different women, and the time of conception after ovulation is not completely consistent, the due date is only an approximate time, and the specific time of delivery may be 1-2 weeks before or 1-2 weeks after the due date.