Ultrasound is the most widely used test in obstetrics to diagnose fetal malformations, abnormal development and lesions of the placenta, umbilical cord and amniotic fluid, thus providing an important basis for doctors to make diagnosis and take timely treatment measures. Although ultrasound is very necessary, there is no authoritative conclusion on the safety of ultrasound in the medical field, and most scholars believe that ultrasound has no definite harm to the fetus. To date, there have been no reports of fetal malformations caused by ultrasound. However, this does not mean that ultrasound can be done as often as you like throughout your pregnancy, without any restrictions on the time and number of times; after all, ultrasound is radioactive, and the longer the ultrasound irradiates the embryo, the greater the adverse effects will be. Therefore, it is not advisable to have multiple ultrasound examinations during pregnancy, and it is generally appropriate to have no more than 4 times during the whole pregnancy. These four times are: 1. In early pregnancy, after 6 weeks of menopause, in addition to the routine gynecological examination, ultrasound should be used to determine whether the intrauterine pregnancy is normal; 2. In the middle of pregnancy, about 20-24 weeks, another ultrasound is needed to understand the general situation of fetal growth and development, so as to understand whether there are abnormalities in the fetus; 3. The ultrasound can clearly understand the growth and development of fetal tissues and organs, so as to understand whether the fetus has malformations; 4. In late pregnancy, ultrasound is done between 36 weeks and the expected date of delivery, which can understand the amount of amniotic fluid and the function of the placenta, as well as whether the fetus has the umbilical cord around the neck, and estimate the weight of the fetus according to the measurement of the fetal head diameter and bones, determine the fetal position, and predict whether the mother-to-be can deliver naturally.