Due to the advancement of technology and the needs of pregnancy, pregnant mothers have to undergo several ultrasounds during their pregnancy. Is ultrasound harmful to the fetus? When is the right time to have an ultrasound? This is a concern for pregnant mothers. However, if the sound waves are dense in a fixed place, and gathered for a long time, there will be a thermal effect, this thermal effect reaches a certain level, may have adverse effects on human tissue, affecting the substances in the cells, including chromosomes. The theory is that high-intensity ultrasound can harm tissues through its high temperature and its cavitation effect on tissues. In fact, medical use of ultrasound is low intensity, less than 94 milliwatts per cubic centimeter, which is not harmful to the fetus, and there have been no reports of fetal malformations caused by ultrasound examinations. Currently, ultrasound used in the field of obstetrics in various hospitals is safe for the fetus. The experiment analysis: use ultrasound only when necessary. Having said that, a trial was done to divide the pregnant women preparing for abortion into two groups, one group did ultrasound before the operation and then performed abortion, and the other group did ultrasound after a week and then did abortion. The pathology found that the embryos with ultrasound had chorionic villus congestion and edema, but the congestion and edema would disappear after a week. The World Health Organization suggests that ultrasound should be used only when necessary and that the fetus should not be irradiated without good reason. The American ultrasound institute proposes that ultrasound should not be used as a diagnostic tool for early pregnancy. Ultrasound in early pregnancy should be indicated to verify the week of pregnancy or to clarify intrauterine pregnancy, for example, to determine whether the pregnancy is ectopic or intrauterine and whether the embryo is viable; to determine if the embryo is located in an intrauterine pregnancy, except for ectopic pregnancy, if there is vaginal bleeding or abdominal pain after pregnancy; or to determine if there is a preterm abortion or fetal arrest, when ultrasound is needed to identify it. Otherwise, ultrasound should not be done routinely. The fetal heartbeat can be seen on general ultrasound at 5 weeks of gestation in utero. The fetus is in the intrauterine period of 3 to 8 weeks when the organs are differentiating, which is a sensitive period for organ differentiation, so unnecessary external stimulation should be reduced. However, ultrasound at about 10 weeks of pregnancy is quite important to verify the gestational week, which is accurate for pregnant women with irregular menstrual cycles and unclear last menstrual period. It provides a reference of gestational week for the diagnosis of fetal growth restriction in late pregnancy. Ultrasound is done in the middle of pregnancy to identify major fetal malformations such as congenital heart disease, neural tube malformation, limb defects, congenital cleft lip and palate, etc. Generally, it is more appropriate to do ultrasound examination in 22 weeks to 26 weeks of pregnancy, because the fetus is about 20 weeks when the brain is developing rapidly, too early to receive ultrasound because the fetus is developing small, some tissues can not see and in vain, can not achieve the purpose of screening malformations, and at the same time affect the development of the fetal brain. In the late pregnancy, you can have an ultrasound at 32 weeks to understand the development of the fetus in the uterus, the amount of amniotic fluid, the size of the fetus, the presence of umbilical cord problems, etc., so as to give proper guidance for further pregnancy. At 37 weeks of pregnancy, an ultrasound can be done to identify the fetus and find out any problems in fetal development, such as placenta, umbilical cord, fetus, etc., in order to prepare for delivery. There are some abnormalities during pregnancy that require ultrasound, such as vaginal bleeding during pregnancy, doctors need to identify whether it is placenta praevia or placenta praevia; preterm labor, which requires ultrasound to identify; intrauterine growth retardation, which requires finding the cause; early water breakage, fetal heart changes, fetal position unclear, etc. may increase the number of ultrasound during pregnancy.