The first day of 280 days is counted from the first day of the last menstrual period of the pregnant mother, and the fetus grows in the uterine cavity for 280 days, which is 40 weeks, and 4 weeks is one month, which is 10 months. In late pregnancy, 28 days is one month, and 280 days is ten months, which is the origin of the expression “ten months of pregnancy”. The due date calculated in this way is only an approximate estimate, and about 70% of people give birth within 10 days before or after the due date, so the due date is only an approximate estimate of when the baby will be delivered.