The time of ultrasound for pregnant women depends on the week of pregnancy. After the first ultrasound examination is found, in most cases, it will be once every 4 weeks until 28 weeks of pregnancy, once every 2 weeks from 28 to 36 weeks of pregnancy, and once a week after 36 weeks of pregnancy. If the fetus and the pregnant woman have abnormalities, they need to do ultrasound examination at any time, the specific pregnancy weeks and the interval between ultrasound examinations are as follows: 1. Early pregnancy: at 6-8 weeks of pregnancy, the pregnant woman should go to the hospital for 1-2 ultrasound examinations to clarify whether the fetus is in the uterine cavity and is alive, so as to exclude ectopic pregnancy and abortion, and to determine whether there is a fetal heart and fetal bud. In 11-12 weeks of pregnancy, NT examination can be performed through ultrasound for initial teratology; 2. Mid-pregnancy: Generally, ultrasound is done once in 16-22 weeks of pregnancy to assess the growth and development of the fetus and the amniotic fluid, while teratology ultrasound is done between 22-26 weeks of pregnancy, which is a mandatory examination to exclude fetal facial abnormalities, digestive system abnormalities, neurological system abnormalities, urinary system abnormalities and fetal 3.Late pregnancy: 28-36 weeks of pregnancy, it is generally recommended to have an ultrasound at 30-32 weeks to assess the fetal growth and development, fetal position, amniotic fluid volume, placenta and so on, and to review the abnormal content of the major abnormality ultrasound by the way; 4.36 weeks of pregnancy to the expected date of delivery: an ultrasound examination at 37-38 weeks can clarify the fetal position, estimate the fetal weight and observe whether the amniotic fluid volume is moderate. Pregnant women with high-risk factors need to review the ultrasound to observe the amniotic fluid volume when they are close to the due date or beyond the due date. The number of ultrasound examinations may change according to the condition of the pregnant woman and the fetus, so you should follow the doctor’s advice when you have a maternity checkup and go to the hospital for an ultrasound examination in case of any discomfort, such as abdominal pain, vaginal bleeding or abnormal fetal movement.