Ultrasound is a mechanical wave that produces biological effects mainly through thermal, mechanical and cavitation effects. It is known that the physical factor under the condition of constant intensity, the general rule is that as the distance of sound propagation increases, the sound intensity gradually decreases. The sound intensity is inversely proportional to the distance, and the biological effect is directly proportional to the time. The ultrasonic diagnostic instrument currently used in China is mostly 3 to 3.5MHz probe frequency, the average power does not exceed 10mw/cm2, the actual application of ultrasound examination is gliding mobile continuous sweep method, complete a full range of sweeping generally does not exceed 5min, difficult lesions also in about 10min, in the abdomen of pregnant women to show the fetus an organ section of a shorter time, time and dose than the fixed irradiation 30 times smaller The time and dose are about 30 times smaller than that of fixed-point irradiation. Therefore, the clinical routine ultrasound scan of pregnant women is much safer for the fetus. In the decades of diagnostic ultrasound use, no evidence of any harmful effects of diagnostic ultrasound equipment on patients has been found, and in this sense, obstetric ultrasound is safe. In the clinical application of diagnostic ultrasound for ultrasound scanning of pregnant women, the principle of ALARA should be adhered to, so that ultrasound can be used in obstetrics as safely and reliably as possible, and comprehensive information on fetal development can be obtained, so that ultrasound can better serve eugenics and play a greater role in reducing the birth rate of infants with disabilities. The ALARA (as low as reasonabl achievable) principle proposed by the American Institute of Ultrasound in Medicine and other organizations is now widely accepted. This principle requires that the examination be completed with the lowest possible sound intensity and in the shortest possible time, while ensuring that the necessary diagnostic ultrasound information is obtained. The appropriate sound intensity depends on many factors, including the patient’s condition and the need for diagnosis. If the sound intensity is too low, good diagnostic information will not be obtained and additional examination time or repeat examinations may be required. If the sound intensity is too high, the diagnostic information obtained will not be improved and the patient is exposed to too much ultrasound. The examination time depends on the diagnostic purpose, the mode of operation, the selection of the probe, and on the experience and skill of the user.