Ultrasound is a mechanical wave, when the sound waves propagate in human tissues, due to the acoustic conduction resistance and friction of tissues, the ultrasound energy can be changed into heat energy, which may cause tissue heating and changes in the function and structure of the organism, but whether the heat production and cavitation effects of ultrasound are produced in the human body depends on the power and frequency of the instrument used, and now the power of the ultrasound diagnostic instrument is 10 microwatts/square centimeter (ultrasound therapy instrument is 0.5~2.5 watts/cm2). According to domestic and foreign experimental studies, ultrasound diagnosis is safe and non-damaging to human body, but the examination time for fetus should not be too long.