Background: The number of days of pregnancy is a matter of great concern to pregnant women and doctors. In the past, days to pregnancy were usually calculated by predicting the start of your last menstrual period. However, this method is not accurate because some women may have irregular menstrual cycles. Now, ultrasound abdominal examination (B-ultrasound) has become the best choice for measuring the number of days of pregnancy. B-ultrasound can not only determine whether pregnancy has occurred, but also determine the pregnancy cycle and expected date of delivery. The following will introduce in detail how to use B-ultrasound to calculate accurate