The due date is 40 weeks of pregnancy, and most people will go into labor naturally around the due date. The calculation of the due date is based on the first day of the last menstrual period, first of all, the formula for calculating the month is to subtract 3 or add 9; the formula for calculating the date is to add 7. If the last menstrual period is January 01, 2020, the month of the due date is 1 plus 3, which is the month of April, and the date is 1 plus 7, which is the 8th, so the due date is April 08, 2020. If the interval between the pregnancy week projected according to the last menstrual period and the pregnancy week projected by early ultrasound is more than 5 days, the pregnancy week projected by early ultrasound should prevail. In early pregnancy, the week of pregnancy is most accurately estimated by the head and hip length of the fetus on ultrasound. The way to calculate the week of pregnancy is to add 6.5 centimeters of head and hip length to the week of pregnancy. If the head and hip length is 6.5 centimeters, it is 13 weeks.