Ultrasound is generally more accurate in measuring the time of pregnancy. In the early stage of pregnancy, ultrasound relies on measuring the size of the gestational sac and fetal bud to project the time of pregnancy. After measuring the value, the software that comes with the ultrasound will substitute into the formula for calculation and then automatically give the time of pregnancy, this formula is derived after a lot of research and is more accurate, the error is usually no more than 3-5 days. In the middle and late pregnancy, due to the differences of the pregnant women themselves, such as height, weight and diet, there will be differences in the individual fetus, so the ultrasound will be inaccurate in calculating the pregnancy time, but the error will not exceed 10 days. Therefore, in order to reduce the error, it is necessary to take multiple measurements and multiple parts of the body to give as many data as possible and then take the average, so that the projected pregnancy time is more accurate.