The easiest way to calculate the due date is to add 9 or subtract 3 to the month of the last menstrual period and add 7 to the day. For example, if a pregnant woman’s last menstrual period is February 2, the expected date of birth is 2 on the basis of 9, that is, November; the day plus 7, the day is 2 is 2 + 7 = 9. Therefore, for a pregnant woman whose last menstrual period was February 2nd, her due date is November 9th. This test is applicable to the vast majority of pregnant women, but there is an important prerequisite that the woman’s periods are relatively regular. In the early stages of pregnancy, the week of pregnancy calculated by ultrasound should be compared with the week of pregnancy calculated by the last menstrual period. If the two are generally consistent, the due date calculated by the last menstrual period is accurate.