Ultrasound is a clear, convenient, and safer way to examine the fetus. But when is an ultrasound needed after pregnancy? Ultrasound examinations are performed at different times for each stage of pregnancy because of the different observations and the different abnormalities that occur at each stage of pregnancy. In general, ultrasound examinations during pregnancy can be performed in four stages: 10-14 weeks gestation stage, 20-24 weeks gestation stage, 27-32 weeks gestation stage, and full-term gestation stage. 1. 10-14W gestational stage: the cases that need to be examined include those who have irregular menstruation or forget the time of menopause and want to know if they are pregnant or estimate the size of the gestational week; those who have unexplained bleeding and abdominal pain after pregnancy; those who suspect gravidity; and before abortion or medical abortion. For those who do not have any clinical symptoms, ultrasound examination can also be temporarily excluded. 2.20-24W gestational stage: At this stage, the fetal limbs have been completely differentiated and the amount of amniotic fluid is relatively high, compared with other gestational weeks, the rate of fetal limbs showing at this stage is higher. In addition, 22-28 weeks of gestation is also the easiest time to obtain a clear picture of the fetal heart. Therefore, those who want to know the development of fetal extremities and heart can have ultrasound examination at this stage. 3.27-32W gestation stage: The contents of fetal observation in this stage are basically the same as those in 20-24 weeks of gestation, except for fetal limbs. In addition, the fetal facial structures can be observed, and the observation of the fetal heart remains the focus. At this stage of ultrasound examination, it is possible to make a more definite diagnosis of some serious fetal malformations. Therefore, for those who want to know more about the development of the fetal limbs and heart, as well as those who need to know the condition of the fetal face and whether the fetus has serious malformations, they can arrange for examination at this stage. 4.Full-term pregnancy stage: after 28 weeks of gestation, it is the best time to examine the internal organs of the fetus, and 36 weeks is the best time for comprehensive examination of the fetus and its appendages. However, when the fetus is developing near full term, the fetal body enlarges and fills the uterine cavity, it is not easy to obtain clear and satisfactory images. At this stage, it is a more appropriate time for the need to know the number of fetuses, fetal orientation, fetal weight; the amount of amniotic fluid, the maturity of the placenta; and whether there are any abnormalities in fetal development. In some special cases, ultrasound examination should be performed at any time or periodically regardless of the stage of pregnancy, for example, suspected ectopic pregnancy; abnormal laboratory tests (elevated or decreased methemoglobin, elevated β-chondroitin, elevated free estriol, positive pregnancy-related protein, etc.); those with obvious teratogenic factors (having taken potentially teratogenic drugs, having been exposed to radiation or harmful substances to the human body, etc.); those with hereditary diseases or family history of hereditary diseases; history of infection during pregnancy (rubella, cytomegalovirus infection, etc.); previous pregnancies with structurally abnormal fetuses (congenital heart disease, etc.); pregnant women with diabetes or other diseases; suspected fetal death; uterine size does not correspond to the time of pregnancy; suspected intrauterine growth retardation; suspected fetal abnormalities, amniotic fluid, placenta or umbilical cord abnormalities; considered twin or multiple The fetal growth and development of the fetus, assessment of fetal weight, etc.; biophysical evaluation of the intrauterine status of the fetus; determination of fetal previa and fetal position; diagnosis of cervical maturity; follow-up observation of normal or abnormal fetus, etc.