The last menstrual period was on March 7 and the due date is expected to be on December 14th. The due date can only be calculated by the method of the last menstrual period for women with regular periods, which is usually calculated according to the menstrual cycle of 28 to 30 days, and projected from the first day of the last menstrual period. The method of calculation is to add 9 or subtract 3 from the month of the last menstrual period, and add 7 to the date, so 3 plus 9 equals 12, 7 plus 7 equals 14, and the expected date of delivery will be December 14 for the last menstrual period on March 7th. For women with less regular periods, ovulation is uncertain, so it is not known when the sperm and egg unite. The estimated due date according to the above method will be a bit off. Therefore, it is necessary to combine the ultrasound size to determine the time of conception of the fetus and then make a projection of the expected date of delivery.