The normal due date is 10 gestational months, or 40 weeks, counting from the first day of the last menstrual period, and the day of the 40th week is the due date, which is the 280th day. 7 days is 1 week, and 4 weeks is one gestational month, which is the 10th month is the day of the due date. The approximate formula for calculating the due date is month-3+9, date+7. This formula is for the days of the solar calendar. If you remember that the first day of your last menstrual period is on the lunar calendar, you need to check the calendar to convert the date to the Gregorian calendar (solar calendar) so that the due date is calculated scientifically. The usual way to calculate the expected date of delivery is to start from the first day of the last menstruation, but this is for pregnant women with a menstrual cycle of 28 days, if the menstruation is irregular, usually between 35-40 days but also correspondingly converted into a menstrual cycle of 28 days, and then apply the formula for calculation.