Ultrasound estimation of fetal weight, usually as a prognosis of fetal weight, is based on the fetal weight measured by ultrasound examination results, usually through the fetal biparietal diameter, head circumference, abdominal circumference, femur length and other developmental indicators, calculated by a specific formula, due to certain clinical factors can affect the measurement data, the calculated fetal weight has a certain error with the actual fetal weight, so it is not an accurate value, but an interval range, but The range of error is not large, within 500g. If the fetal development is more balanced, the difference between the calculated value and the actual weight will be smaller. Therefore, the weight estimated by ultrasound can be used as a reference, but it cannot be determined as the actual weight of the fetus. The reasons for the error are as follows: 1) fetal subcutaneous fat: different fetuses have different subcutaneous fat, which cannot be accurately measured by ultrasound and can cause measurement error; 2) fetal position: different positions of the fetus in the uterus can affect the accuracy of the data, and if the fetus is in a curled position, it can cause measurement error; 3) maternal body type: the weight of the pregnant woman, the thickness of abdominal fat, the size and structure of the skeleton can all cause measurement error. The measurement error can be caused by the different weight, abdominal fat thickness, skeleton size and structure of the pregnant woman. Therefore, ultrasonographers are required to try to obtain accurate values during the examination, and to perform repeated measurements and measure as many indicators as possible to reduce the error in weight estimation by ultrasound.