The actual date of delivery may differ from the estimated due date by 1-2 weeks. For pregnant women with irregular menstrual cycles, breastfeeding pregnancies or those who do not remember their last menstrual period, the expected date of delivery can be estimated based on ultrasound in early pregnancy, which includes the size of the fetal sac, the length of the head and hip diameter, the biparietal diameter and the length of the femur. Sometimes, the expected date of delivery can also be estimated based on the time of early pregnancy reaction, the time when the pregnant woman feels the fetal movement, and the height of the uterine fundus. Ultrasound within 14 weeks of gestation is the most accurate way to determine the due date and gestational age. If there is a significant difference between the ultrasound due date and the due date projected from the last menstruation, the ultrasound due date should be used. Once the due date is determined, do not change it easily to avoid confusion or mishandling.