The due date is calculated based on the first day of the last menstrual period. The formula is the month of the first day of the last menstrual period plus 9 months or minus 3 months, which is the number of months of the due date, and the date plus 7 days, which is the day of the due date. The whole pregnancy period is 280 days, 40 weeks and 10 gestational months. However, the due date is an approximate time of delivery and is not an exact date of delivery. Since women’s menstrual cycles are not exactly the same, the time of ovulation is not exactly fixed, the time of intercourse is not fixed, and the exact time of conception cannot be accurately determined, there is a certain difference between the expected date of delivery and the actual date of delivery, which is usually about 1-2 weeks. If a pregnant woman’s menstrual cycle is not fixed, she can go to the hospital for an ultrasound to measure the CRL value or determine the gestational week based on the HCG value of the early pregnancy test. Because screening for Down syndrome is performed based on the week of pregnancy, it is more important to check for an accurate due date since an incorrect calculation of the week of pregnancy may result in a high-risk or borderline risk screening result. When a pregnant woman learns that she is pregnant, she should go to the hospital for a timely checkup to get a more accurate due date and also to check if the fetus is developing normally.