For women with normal menstrual cycles, the actual number of days of pregnancy is calculated from the first day of the last menstrual period. For women who usually have irregular menstrual cycles, the actual number of days of pregnancy can be estimated from early ultrasound. If the number of days of pregnancy is calculated by early ultrasound, it can be based on the size of the gestational sac, and the average value of the millimeter of the size of the gestational sac plus 25 is the number of days of pregnancy; it can also be based on the size of the embryo, and the size of the embryo in centimeters plus 42 is the number of days of pregnancy; and it can also be based on the circumference of the fetus’s head and hips, and the circumference in centimeters of the head and hips plus 6.5, which is the number of weeks of pregnancy. For pregnant women who are more than 12 weeks pregnant, the number of days of pregnancy can also be calculated based on the biparietal diameter and femoral diameter of the fetus, however, the number of days of pregnancy based on the ultrasound after 12 weeks of pregnancy, the error is larger, and it is recommended that the number of days of pregnancy be calculated based on the data from the ultrasound before the 12th week of pregnancy.