Pregnant women can often do ultrasound, ultrasound refers to the frequency of sound waves above 20,000 Hz, inaudible to the human ear, the principle of ultrasound instruments is mainly based on the different densities of various tissues and organs, sound waves into the body to produce different reflections, refraction, absorption, attenuation, through the reflected waves in the ultrasound above the internal organ images can be displayed, so it belongs to a physical examination method, its emission mode is intermittent pulse Emission. Ultrasound has been used in clinical practice for 40 years, and its safety has been confirmed. Theoretically, high-intensity ultrasound may cause damage to tissues, but the ones used in medicine are of low intensity, so it does not have radiation and biological effects like X-rays, and generally does not cause harm to pregnant women and fetuses, with the benefits of safety, simplicity and non-invasiveness, and can be used repeatedly. No fetal or tissue damage has been reported so far.