How to accurately calculate the expected date of delivery Accurate calculation of the expected date of delivery is crucial for expectant mothers. It is the basis of pregnancy management and can help pregnant women make various preparations in time. There are many methods for calculating the due date. We can understand these methods through the following aspects. First of all, the mother’s last menstrual period is an important reference factor in calculating the expected date of delivery. The expected date of delivery is calculated by adding 280 days to the first day of the last menstrual period. For example, if the first day of your period is July 1, then your due date is April 7. However, this method is only an estimate and cannot predict completely accurately.