The number of weeks of gestation is calculated from the first day of the last menstrual period, and seven days is a week, usually 40 weeks of gestation, which is approximately 280 days. The estimated due date is calculated from the first day of the last menstrual period, the month minus three or plus nine, and the number of days plus seven. If possible, you should check the expected date of delivery based on the ultrasound report of early pregnancy, especially if you are pregnant with unclear last menstruation, no menstruation during breastfeeding, separated from each other or irregular menstruation. If there is a difference of more than five days between the gestational week projected from the last menstrual period and the early ultrasound test, the due date should be checked according to the early ultrasound of pregnancy, which is the most correct indicator to estimate the gestational week.